# 【RFC文档】RFC 1195 英文 – Use of OSI IS-IS for routing in TCP/IP and dual environments

RFC 1195

Document	Type		RFC - Proposed Standard (December 1990; Errata)
Updated by RFC 5302, RFC 5304, RFC 1349
Last updated	2013-03-02
Stream		Legacy

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IESG	        IESG state		RFC 1195 (Proposed Standard)
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Network Working Working Group                                  R. Callon
Request for Comments: 1195                 Digital Equipment Corporation
December 1990

Use of OSI IS-IS for Routing in TCP/IP and Dual Environments

Status of this Memo

This RFC specifies a protocol on the IAB Standards Track for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "IAB
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.

This RFC is available in both postscript and text versions. Where
possible, use of the postscript version is recommended. For example,
this text version may have figures which are less informative or
missing.

Abstract

This RFC specifies an integrated routing protocol, based on the OSI
Intra-Domain IS-IS Routing Protocol, which may be used as an interior
gateway protocol (IGP) to support TCP/IP as well as OSI. This allows
a single routing protocol to be used to support pure IP environments,
pure OSI environments, and dual environments. This specification was
developed by the IS-IS working group of the Internet Engineering Task
Force.

The OSI IS-IS protocol has reached a mature state, and is ready for
OSI IS-IS protocol is contained in ISO DP 10589 [1]. The proposed
standard for using IS-IS for support of TCP/IP will therefore make
use of this version (with a minor bug correction, as discussed in
Annex B).  We expect that future versions of this proposed standard
will upgrade to the final International Standard version of IS-IS
when available.

Comments should be sent to "isis@merit.edu".

Contents

1   Introduction: Overview of the Protocol
1.1     What the Integrated IS-IS offers
1.2     Overview of the ISO IS-IS Protocol
1.3     Overview of the Integrated IS-IS
1.4     Support of Mixed Routing Domains

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

1.5     Advantages of Using Integrated IS-IS

2   Symbols and Abbreviations

3   Subnetwork Independent Functions
3.1     Exchange of Routing Information
3.2     Hierarchical Abbreviation of IP Reachability Information
3.3     Addressing Routers in IS-IS Packets
3.5     Type of Service Routing
3.6     Multiple LSPs and SNPs
3.7     IP-Only Operation
3.8     Encapsulation
3.9     Authentication
3.10    Order of Preference of Routes / Dijkstra Computation

4   Subnetwork Dependent Functions
4.2     Multiple IP Addresses per Interface
4.3     LANs, Designated Routers, and Pseudonodes
4.5     Forwarding to Incompatible Routers

5   Structure and Encoding of PDUs
5.1     Overview of IS-IS PDUs
5.2     Overview of IP-Specific Information for IS-IS
5.3     Encoding of IP-Specific Fields in IS-IS PDUs

6   Security Considerations

8   References

A   Inter-Domain Routing Protocol Information
A.1     Inter-Domain Information Type
A.2     Encoding

B   Encoding of Sequence Number Packets
B.1     Level 1 Complete Sequence Numbers PDU
B.2     Level 2 Complete Sequence Numbers PDU
B.3     Level 1 Partial Sequence Numbers PDU
B.4     Level 2 Partial Sequence Numbers PDU

C   Dijkstra Calculation and Forwarding
C.1     SPF Algorithm for IP and Dual Use
C.2     Forwarding of IP packets

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D   Use of the Authentication Field
D.1     Authentication Field in IS-IS packets
D.2     Authentication Type 1 - Simple Password

E   Interaction of the Integrated IS-IS with Brouters
E.1     The Problem
E.2     Possible Solutions

Figures
2       An Example
3       Encoding of Variable Length Fields

1 Introduction: Overview of the Protocol

The TCP/IP protocol suite has been growing in importance as a multi-
vendor communications architecture. With the anticipated emergence of
OSI, we expect coexistence of TCP/IP and OSI to continue for an
extended period of time. There is a critical need for routers to
support both IP traffic and OSI traffic in parallel.

There are two main methods that are available for routing protocols
to support dual OSI and IP routers. One method, known as "Ships in
the Night", makes use of completely independent routing protocols for
each of the two protocol suites. This specification presents an
alternate approach, which makes use of a single integrated protocol
for interior routing (i.e., for calculating routes within a routing
domain) for both protocol suites.

This integrated protocol design is based on the OSI Intra-domain IS-
IS routing protocol [1], with IP-specific functions added. This RFC
is considered a companion to the OSI IS-IS Routing spec, and will
only describe the required additional features.

By supporting both IP and OSI traffic, this integrated protocol
design supports traffic to IP hosts, OSI end systems, and dual end
systems.  This approach is "integrated" in the sense that the IS-IS
protocol can be used to support pure-IP environments, pure-OSI
environments, and dual environments. In addition, this approach
allows interconnection of dual (IP and OSI) routing domains with
other dual domains, with IP-only domains, and with OSI-only domains.

The protocol specified here is based on the work of the IETF IS-IS
working group.

1.1 What the Integrated IS-IS offers

The integrated IS-IS provides a single routing protocol which will

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simultaneously provide an efficient routing protocol for TCP/IP, and
for OSI. This design makes use of the OSI IS-IS routing protocol,
augmented with IP-specific information. This design provides explicit
support for IP subnetting, variable subnet masks, TOS-based routing,
and external routing. There is provision for authentication
information, including the use of passwords or other mechanisms. The
precise form of authentication mechanisms (other than passwords) is
outside of the scope of this document.

Both OSI and IP packets are forwarded "as is" -- i.e., they are
transmitted directly over the underlying link layer services without
the need for mutual encapsulation. The integrated IS-IS is a dynamic
routing protocol, based on the SPF (Dijkstra) routing algorithm.

The protocol described in this specification allows for mixing of
IP-only, OSI-only, and dual (IP and OSI) routers, as defined below.

An IP-only IS-IS router (or "IP-only" router) is defined to be a
router which: (i) Uses IS-IS as the routing protocol for IP, as
specified in this report; and (ii) Does not otherwise support OSI
protocols. For example, such routers would not be able to forward OSI
CLNP packets.

An OSI-only router is defined to be a router which uses IS-IS as the
routing protocol for OSI, as specified in [1]. Generally, OSI-only
routers may be expected to conform to OSI standards, and may be
implemented independent of this specification.

A dual IS-IS router (or "dual" router) is defined to be a router
which uses IS-IS as a single integrated routing protocol for both IP
and OSI, as specified in this report.

This approach does not change the way that IP packets are handled.
IP-only and dual routers are required to conform to the requirements
of Internet Gateways [4]. The integrated IS-IS protocol described in
this report outlines an Interior Gateway Protocol (IGP) which will
provide routing within a TCP/IP routing domain (i.e., autonomous
system). Other aspects of router functionality (e.g., operation of
ICMP, ARP, EGP, etc.) are not affected by this proposal.

Similarly, this approach does not change the way that OSI packets are
handled. There will be no change at all to the contents nor to the
handling of ISO 8473 Data packets and Error Reports, nor to ISO 9542
Redirects and ES Hellos. ISO 9542 IS Hellos transmitted on LANs are
similarly unchanged. ISO 9542 IS Hellos transmitted on point-to-point
information.  Similarly, other OSI packets (specifically those
involved in the IS-IS intra-domain routing protocol) remain unchanged

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except for the addition of IP-related information.

This approach makes use of the existing IS-IS packets, with IP-
specific fields added. Specifically: (i) authentication information
may be added to all IS-IS packets; (ii) the protocols supported by
each router, as well as each router's IP addresses, are specified in
ISO 9542 IS Hello, IS-IS Hello and Link State Packets; (iii)
Packets; and (iv) externally reachable IP addresses, and external
routing protocol information, may be specified in level 2 Link State
Packets. The detailed encoding and interpretation of this in
formation is specified in sections 3, 4, and 5 of this RFC.

The protocol described in this report may be used to provide routing
in an IP-only routing domain, in which all routers are IP-only.
Similarly, this protocol may be used to provide routing in a pure
dual domain, in which all routers are dual. Finally, this protocol
may be used to provide routing in a mixed domain, in which some
routers are IP-only, some routers are OSI-only, and some routers are
dual. The specific topological restrictions which apply in this
latter case are described in detail in section 1.4 ("Support of Mixed
Routing Domains").  The use of IS-IS for support of pure OSI domains
is specified in [1].

This protocol specification does not constrain which network
management protocol(s) may be used to manage IS-IS-based routers.
Management information bases (MIBs) for managing IP-only, OSI-only,
and dual routers, compatible with CMIP, CMOT, and/or SNMP, are the
subject of a separate, companion document [8].

1.2 Overview of the ISO IS-IS Protocol

The IS-IS Routing Protocol has been developed in ISO to provide
routing for pure OSI environments. In particular, IS-IS is designed
to work in conjunction with ISO 8473 (The ISO Connectionless Network
Layer Protocol [2]), and ISO 9542 (The ISO End System to Intermediate
System Protocol [3]). This section briefly describes the manner in
which IS-IS is used to support pure OSI environments. Enhancements
for support of IP and dual environments are specified elsewhere in
this report.

In IS-IS, the network is partitioned into "routing domains". The
boundaries of routing domains are defined by network management, by
"exterior", no IS-IS routing messages are sent on that link.

Currently, ISO does not have a standard for inter-domain routing
(i.e., for routing between separate autonomous routing domains).

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and with the method by which they can be reached (such as the DTE
address to be dialed to reach that address, or the fact that the DTE
address should be extracted from the IDP portion of the ISO address).

OSI IS-IS routing makes use of two-level hierarchical routing. A
routing domain is partitioned into areas. Level 1 routers know the
topology in their area, including all routers and end systems in
their area. However, level 1 routers do not know the identity of
routers or destinations outside of their area. Level 1 routers
forward all traffic for destinations outside of their area to a level
2 router in their area. Similarly, level 2 routers know the level 2
topology, and know which addresses are reachable via each level 2
router. However, level 2 routers do not need to know the topology
within any level 1 area, except to the extent that a level 2 router
may also be a level 1 router within a single area. Only level 2
routers can exchange data packets or routing information directly
with external routers located outside of the routing domains.

+----------------------+-------------------------------+
|        IDP           |              DSP              |
+----------------------+-------------------------------+
.                      .                               .
.                      .                               .
.                      .                               .
+-----+----------------+----------+--------------+-----+
| AFI |      IDI       |  HO-DSP  |      ID      | SEL |
+-----+----------------+----------+--------------+-----+

Figure 1 - ISO Hierarchical Address Structure

As illustrated in figure 1, ISO addresses are subdivided into the
Initial Domain Part (IDP), and the Domain Specific Part (DSP). The
IDP is the part which is standardized by ISO, and specifies the
format and authority responsible for assigning the rest of the
specified by the IDP. The DSP is further subdivided into a "High
Order Part of DSP" (HO-DSP), a system identifier (ID), and an NSAP
selector (SEL). The HO-DSP may use any format desired by the
authority which is identified by the IDP. Together, the combination
of [IDP, HO-DSP] identify both the routing domain and the area within
the routing domain. The combination of [IDP,HO-DSP] may therefore be
referred to as the "Area Address".

Usually, all nodes in an area have the same area address. However,
sometimes an area might have multiple addresses. Motivations for

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allowing this are:

- It might be desirable to change the address of an area. The most
graceful way of changing an area from having address A to having
address B is to first allow it to have both addresses A and B, and
then after all nodes in the area have been modified to recognize
both addresses, then one by one the nodes can be modified to

- It might be desirable to merge areas A and B into one area. The
method for accomplishing this is to, one by one, add knowledge of
address A into the B  partition.

- It might be desirable to partition an area C into two areas, A
and B (where "A" might equal "C", in which case this example
becomes one of removing a portion of an area). This would be
accomplished by first introducing knowledge of address A into
the appropriate nodes (those destined to become area A), and
knowledge of address B into the appropriate nodes, and then one
by one removing knowledge of address C.

Since OSI addressing explicitly identifies the area, it is very easy
for level 1 routers to identify packets going to destinations outside
of their area, which need to be forwarded to level 2 routers.

In IS-IS, there are two types of routers:

- Level 1 intermediate systems -- these nodes route based on the ID
portion of the ISO address. They route within an area. They
recognize, based on the destination address in a packet, whether
the destination is within the area. If so, they route towards
the destination. If not, they route to the nearest level 2 router.

- Level 2 intermediate systems -- these nodes route based on the area
address (i.e., on the combination of [IDP, HO-DSP]). They route
towards areas, without regard to the internal structure of an area.
A level 2 IS may also be a level 1 IS in one area.

A level 1 router will have the area portion of its address manually
configured. It will refuse to become a neighbor with a node whose
area addresses do not overlap its area addresses. However, if level 1
router has area addresses A,  B, and C, and a neighbor has area
addresses B and D, then the level 1 router will accept the other node
as a neighbor.

A level 2 router will accept another level 2 router as a neighbor,

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overlap, the link would be considered by both routers to be "level 2
only", and only level 2 LSPs would flow on the link. External links
(to other routing domains) must be from level 2 routers.

IS-IS provides an optional partition repair function. In the unlikely
case that a level 1 area become partitioned, this function, if
implemented, allows the partition to be repaired via use of level 2
routes.

IS-IS requires that the set of level 2 routers be connected. Should
the level 2 backbone become partitioned, there is no provision for
use of level 1 links to repair a level 2 partition.

In unusual cases, a single level 2 router may lose connectivity to
the level 2 backbone. In this case the level 2 router will indicate
in its level 1 LSPs that it is not "attached", thereby allowing level
1 routers in the area to route traffic for outside of the domain to a
different level 2 router. Level 1 routers therefore route traffic to
destinations outside of their area only to level 2 routers which
indicate in their level 1 LSPs that they are "attached".

An end system may autoconfigure the area portion of its address by
extracting the area portion of a neighboring router's address. If
this is the case, then an endnode will always accept a router as a
neighbor. Since the standard does not specify that the end system
MUST autoconfigure its area address, an end system may be configured
with an area address. In this case the end system would ignore router

Special treatment is necessary for broadcast subnetworks, such as
LANs. This solves two sets of issues: (i) In the absence of special
treatment, each router on the subnetwork would announce a link to
every other router on the subnetwork, resulting in n-squared links
reported; (ii) Again, in the absence of special treatment, each
router on the LAN would report the same identical list of end systems
on the LAN, resulting in substantial duplication.

These problems are avoided by use of a "pseudonode", which represents
the LAN. Each router on the LAN reports that it has a link to the
pseudonode (rather than reporting a link to every other router on the
LAN). One of the routers on the LAN is elected "designated router".
The designated router then sends out an LSP on behalf of the
pseudonode, reporting links to all of the routers on the LAN. This
the pseudonode LSP includes the list of end systems on the LAN,
thereby eliminating the potential duplication (for further
information on designated routers and pseudonodes, see [1]).

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The IS-IS provides for optional Quality of Service (QOS) routing,
based on throughput (the default metric), delay, expense, or residual
error probability. This is described in greater detail in section
3.5, and in [1].

1.3 Overview of the Integrated IS-IS

The integrated IS-IS allows a single routing protocol to be used to
route both IP and OSI packets. This implies that the same two-level
hierarchy will be used for both IP and OSI routing. Each area will be
specified to be either IP-only (only IP traffic can be routed in that
particular area), OSI-only (only OSI traffic can be routed in that
area), or dual (both IP and OSI traffic can be routed in the area).

This proposal does not allow for partial overlap of OSI and IP areas.
For example, if one area is OSI-only, and an other area is IP-only,
then it is not permissible to have some routers be in both areas.
Similarly, a single backbone is used for the routing domain. There is
no provision for independent OSI and IP backbones.

Similarly, within an IP-only or dual area, the amount of knowledge
maintained by routers about specific IP destinations will be as
similar as possible as for OSI. For example, IP-capable level 1
routers will maintain the topology within the area, and will be able
to route directly to IP destinations within the area. However, IP-
capable level 1 routers will not maintain information about
destinations outside of the area. Just as in normal OSI routing,
traffic to destinations outside of the area will be forwarded to the
nearest level 2 router. Since IP routes to subnets, rather than to
specific end systems, IP routers will not need to keep nor distribute
lists of IP host identifiers (note that routes to hosts can be
announced by using a subnet mask of all ones).

The IP address structure allows networks to be partitioned into
subnets, and allows subnets to be recursively subdivided into smaller
subnets. However, it is undesireable to require any specific
relationship between IP subnet addresses and IS-IS areas. For
example, in many cases, the dual routers may be installed into
existing environments, which already have assigned IP and/or OSI
be assigned. We therefore will not require any specific relationship
between IP addresses and the area structure. The IP addresses can be
assigned completely independently of the OSI addresses and IS-IS area
structure. As will be described in section 3.2 ("Hierarchical
Abbreviation of IP Reachability Information"), greater efficiency and
scaling of the routing algorithm can be achieved if there is some
correspondence between the IP address assignment structure and the

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

area structure.

Within an area, level 1 routers exchange link state packets which
identify the IP addresses reachable by each router. Specifically,
included in each Link State Packet. Each level 1 router is manually
which are reachable on each interface. A level 1 router routes as
follows:

mask, metric] reachable within the area, the packet is routed via
level 1 routing.

- If a specified destination address does not match any [IP address,
subnet mask, metric] combination listed as reachable within the
area, the packet is routed towards the nearest level 2 router.

Flexible use of the limited IP address space is important in order to
cope with the anticipated growth of IP environments. Thus an area
(and by implication a routing domain) may simultaneously make use of
area (or domain). Generally, if a specified destination address
specific address is the one routed towards (the one with more "1"
bits in the mask -- this is known as "best match" routing).

Level 2 routers include in their level 2 LSPs a complete list of [IP
in their area. As described in section 3, this information may be
obtained from a combination of the level 1 LSPs (obtained from level
1 routers in the same area), and/or by manual configuration. In
addition, Level 2 routers may report external reachability
information, corresponding to addresses which can be reached via
routers in other routing domains (autonomous systems)

Default routes may be announced by use of a subnet mask containing
all zeroes. Default routes should be used with great care, since they
can result in "black holes". Default routes are permitted only at
level 2 as external routes (i.e., included in the "IP External
Reachability Information" field, as explained in sections 3 and 5).
Default routes are not permitted at level 1.

The integrated IS-IS provides optional Type of Service (TOS) routing,
through use of the QOS feature from IS-IS.

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1.4 Support of Mixed Routing Domains

The integrated IS-IS proposal specifically allows for three types of
routing domains:

- Pure IP

- Pure OSI

- Dual

In a pure IP routing domain, all routers must be IP-capable. IP-only
routers may be freely mixed with dual routers. Some fields
specifically related to OSI operation may be included by dual
routers, and will be ignored by IP-only routers. Only IP traffic will
be routed in an pure IP domain. Any OSI traffic may be discarded
(except for the IS-IS packets necessary for operation of the routing
protocol).

In a pure OSI routing domain, all routers must be OSI-capable.  OSI-
only routers may be freely mixed with dual routers. Some fields
specifically related to IP operation may be included by dual routers,
and will be ignored by OSI-only routers. Only OSI traffic will be
routed in a pure OSI domain. Any IP traffic may be discarded.

In a dual routing domain, IP-only, OSI-only, and dual routers may be
mixed on a per-area basis. Specifically, each area may itself be
defined to be pure IP, pure OSI, or dual.

In a pure IP area within a dual domain, IP-only and dual routers may
be freely mixed. Only IP traffic can be routed by level 1 routing
within a pure-IP area.

In a pure-OSI area within a dual domain, OSI-only and dual routers
may be freely mixed. Only OSI traffic can be routed by level 1
routing within a pure OSI area.

In a dual area within a dual routing domain only dual routers may be
used. Both IP and OSI traffic can be routed within a dual area.

Within a dual domain, if both IP and OSI traffic are to be routed
between areas then all level 2 routers must be dual.

1.5 Advantages of Using Integrated IS-IS

Use of the integrated IS-IS protocol, as a single protocol for
routing both IP and OSI packets in a dual environment, has
significant advantages over using separate protocols for

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independently routing IP and OSI traffic.

An alternative approach is known as "Ships In the Night" (S.I.N.).
With the S.I.N. approach, completely separate routing protocols are
used for IP and for OSI. For example, OSPF [5] may be used for
routing IP traffic, and IS-IS [1] may be used for routing OSI
traffic. With S.I.N., the two routing protocols operate more or less
independently. However, dual routers will need to implement both
routing protocols, and therefore there will be some degree of
competition for resources.

Note that S.I.N. and the integrated IS-IS approach are not really
completely separate options. In particular, if the integrated IS-IS
is used within a routing domain for routing of IP and OSI traffic, it
is still possible to use other independent routing protocols for
routing other protocol suites.

In the future, optional extensions to IS-IS may be defined for
routing other common protocol suites. However, such future options
are outside of the scope of this document. This section will compare
integrated IS-IS and S.I.N. for routing of IP and OSI only.

A primary advantage of the integrated IS-IS relates to the network
management effort required. Since the integrated IS-IS provides a
single routing protocol, within a single coordinated routing domain
using a single backbone, this implies that there is less information
to configure. This combined with a single coordinated MIB simplifies
network management.

Note that the operation of two routing protocols with the S.I.N.
approach are not really independent, since they must share common
resources. However, with the integrated IS-IS, the interactions are
explicit, whereas with S.I.N., the interactions are implicit. Since
the interactions are explicit, again it may be easier to manage and
debug dual routers.

Another advantage of the integrated IS-IS is that, since it requires
only one routing protocol, it uses fewer resources. In particular,
less implementation resources are needed (since only one protocol
needs to be implemented), less CPU and memory resources are used in
the router (since only one protocol needs to be run), and less
network resources are used (since only one set of routing packets
need to be transmitted). Primarily this translates into a financial
savings, since each of these three types of resources cost money.
This implies that dual routers based on the integrated IS-IS should
be less expensive to purchase and operate than dual routers based on
S.I.N.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

Note that the operation of two routing protocols with the S.I.N.
approach are not really independent, since they must share common
resources. For example, if one routing protocol becomes unstable and
starts to use excessive resources, the other protocol is likely to
suffer. A bug in one protocol could crash the other. However, with
the integrated IS-IS, the interactions are explicit and are defined
into the protocol and software interactions. With S.I.N., the
interactions are implicit.

The use of a single integrated routing protocol similarly reduces the
likely frequency of software upgrades. Specifically, if you have two
the software any time EITHER of the protocols change. If you make use
of a single integrated routing protocol, then software changes are
still likely to be needed, but less frequently.

Finally, routing protocols have significant real time requirements.
In IS-IS, these real time requirements have been explicitly
specified. In other routing protocols, these requirements are
implicit. However, in all routing protocols, there are real time
guarantees which must be met in order to ensure correct operation. In
general, it is difficult enough to ensure compliance with real time
requirements in the implementation of a single real time system. With
S.I.N., implementation of two semi-independent real-time protocols in
a single device makes this more difficult.

Note that both integrated IS-IS and S.I.N. allow for independence of
external routes (for traffic from/to outside of the routing domain),
and allow for independent assignment of OSI and TCP/IP addresses.

2 Symbols and Abbreviations

(a three octet field in the GOSIP version 2.0 NSAP

AFI             Authority and Format Identifier
(the first octet of all OSI NSAP addresses -- identifies
format of the rest of the address)

CLNP            Connection-Less Network Protocol
(ISO 8473, the OSI connectionless network layer protocol
-- very similar to IP)

DFI             DSP Format Identifier
(a one octet field in the GOSIP version 2.0 NSAP address
format)

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

ES              End System
(The OSI term for a host)

ES-IS           End System to Intermediate System Routeing Exchange
Protocol (ISO 9542 -- OSI protocol between routers
and end systems)

ICD             International Code Designator
(ISO standard for identifying organizations)

IP              Internetwork Protocol
(an Internet Standard Network Layer Protocol)

IS              Intermediate System
(The OSI term for a router)

IS-IS           Intermediate System to Intermediate System Routeing
Exchange Protocol
(the ISO protocol for routing within a single
routing domain)

IS-IS Hello     An Hello packet defined by the IS-IS protocol
(a type of packet used by the IS-IS protocol)

ISH             An Hello packet defined by ISO 9542 (ES-IS protocol).
(not the same as IS-IS Hello)

ISO             International Organization for Standardization
(an international body which is authorized to write
standards of many kinds)

(a type of packet used by the IS-IS protocol)

NLPID           Network Layer Protocol ID
(A one-octet field identifying a network layer protocol)

NSAP            Network Service Access Point
(a conceptual interface point at which the network

SEL             NSAP Selector
(the last octet of NSAP addresses, also called NSEL)

OSI             Open Systems Interconnection
(an international standard protocol architecture)

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

RD              Routing Domain
(the set of routers and end systems using a single
instance of a routing protocol such as IS-IS)

SNPA            Subnetwork Point of Attachment
(a conceptual interface at which a subnetwork service
is provided)

TCP             Transmission Control Protocol
(an Internet Standard Transport Layer Protocol)

TCP/IP          The protocol suite based on TCP, IP, and related
protocols (the Internet standard protocol
architecture)

3 Subnetwork Independent Functions

3.1 Exchange of Routing Information

The exchange of routing information between routers makes use of the
normal routing packet exchange as defined in the OSI IS-IS routing
routing packets.

The IS-IS protocol provides for the inclusion of variable length
fields in all IS-IS packets. These fields are encoded using a "Code,
Length, Value" triplet, where the code and length are encoded in one
octet each, and the value has the length specified (from 0 to 254
octets). IS-IS requires that: "Any codes in a received PDU that are
not recognised are ignored and passed through unchanged". This
requirement applies to all routers implementing IS-IS, including
OSI-only, IP-only, and dual routers. This allows IP-specific
information to be encoded in a manner which OSI-only routers will
ignore, and also allows OSI-specific information to be encoded in a
manner which IP-only routers will ignore.

IP-capable (i.e., all IP-only and dual) routers need to know what
network layer protocols are supported by other routers in their area.
This information is made available by inclusion of a "protocols
supported" field in all IS-IS Hello and Link State Packets. This
field makes use of the NLPID (Network Layer Protocol Identifier),
which is a one-octet value assigned by ISO to identify network level
protocols. NLPID values have been assigned to ISO 8473 and to IP.

interface of neighboring routers. This is required for sending ICMP
redirects (when an IP-capable router sends an ICMP redirect to a
host, it must include the IP address of the appropriate interface of

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the correct next-hop router). This information is made available by
inclusion of the IP interface address in the IS-IS Hello packets.
Specifically, each IS-IS Hello packet contains the IP address(es) of
the interface over which the Hello is transmitted. The IS-IS allows
multiple IP addresses to be assigned to each physical interface.

In some cases, it will be useful for IP-capable routers to be able to
determine an IP address(es) of all other routers at their level
(i.e., for level 1 routers: all other routers in their area; for
level 2 routers: all other level 2 routers in the routing domain).
This is useful whenever an IP packet is to be sent to a router, such
as for encapsulation or for transmission of network management
packets. This information is made available by inclusion of IP
address in LSPs. Specifically, each IS-IS LSP includes one or more IP
addresses of the router which transmits the LSP. An IP-capable router
is required to include at least one of its IP addresses in its LSPs,
and may optionally include several or all of its IP addresses. Where
a single router operates as both a level 1 and a level 2 router, it
is required to include the same IP address(es) in its level 1 and
level 2 LSPs.

IP-capable routers need to know, for any given IP destination
address, the correct route to that destination. Specifically, level 1
routers need to know what IP addresses are reachable from each level
1 router in their area. In addition, level 1 routers need to find
level 2 routers (for traffic to IP addresses outside of their area).
Level 2 routers need to know what IP addresses are reachable
internally (either directly, or via level 1 routing) from other level
2 routers, and what addresses are reachable externally from other
level 2 routers. All of this information is made available by
Packets.

Internal (within the routing domain) and external (outside the
domain) reachability information is announced separately in level 2
LSPs. Reachable IP addresses include a default metric, and may
include multiple TOS-specific metrics. In general, for external
routes, metrics may be of type "internal" (i.e., directly comparable
with internal metrics) or of type "external" (i.e., not comparable
with the internal metric). A route using internal metrics (i.e.,
either announced as "IP internal reachability information", or
announced as "IP external reachability information" with an internal
metric) is always preferred to a route using external metrics (i.e.,
announced as "IP external reachability information", with an external
metric).

The detailed encoding of the IP-specific information included in
routing packets is provided in section 5 (Structure and Encoding of

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PDUs).

3.2 Hierarchical Abbreviation of IP Reachability Information

Level 2 routers include in their level 2 LSPs a list of all [IP
In general, this information may be determined from the level 1 LSPs
from all routers in the area. If we ignore resource constraints, then
it would be permissible for a level 2 router to simply duplicate all
[IP address, subnet mask, metric] entries from all level 1 routers in
its area (with appropriate metric adjustment), for inclusion in its
level 2 LSP. However, in order for hierarchical routing to scale to
large routing domain sizes, it is highly desired to abbreviate the

This is accomplished by manual configuration of summary addresses.
Each level 2 router may be configured with one or more [IP address,
subnet mask, metric] entries for announcement in their level 2 LSPs.

The set of reachable addresses obtained from level 1 LSPs is compared
with the configured reachable addresses. Redundant information
obtained from level 1 LSPs is not included in level 2 LSPs. Generally
it is expected that the level 2 configured information will specify
bits set to 1). This will therefore allow one configured
hierarchically supercede the information corresponding to multiple
entries in level 1 LSPs.

The manually configured addresses are included in level 2 LSPs only
if they correspond to at least one address which is reachable in the
area. For manually configured level 2 addresses, the associated
metric values to announce in level 2 LSPs are also manually
entries from level 1 LSPs based only on the IP address and subnet
mask -- metric values are not considered when determining if a given

Any address obtained from a level 1 LSP which is not superceded by
the manually configured information is included in the level 2 LSPs.
In this case, the metric value announced in the level 2 LSPs is
calculated from the sum of the metric value announced in the
corresponding level 1 LSP, plus the distance from the level 2 router
to the appropriate level 1 router. Note: If this sum results in a
metric value greater than 63 (the maximum value that can be reported
in level 2 LSPs), then the value 63 must be used. Delay, expense, and
error metrics (i.e., those TOS metrics other than the default metric)
will be included only if (i) the level 2 router supports the specific

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TOS; (ii) the path from the level 2 router to the appropropriate
level 1 router is made up of links which support the specific TOS;
and (iii) the level 1 router which can reach the address directly
also supports the specific TOS for this route, as indicated in its
level 1 LSP.

In general, the same [IP address, subnet mask] pair may be announced
in level 1 LSPs sent by multiple level 1 routers in the same area. In
this case (assuming the entry is not superceded by a manually
configured entry), then only one such entry shall be included in the
level 2 LSP. The metric value(s) announced in level 2 LSPs correspond
to the minimum of the metric value(s) that would be calculated for
each of the level 1 LSP entries.

A level 2 router will have IP addresses which are directly reachable
via its own interfaces. For purposes of inclusion of IP reachable
address information in level 2 LSPs, these "directly reachable"
1 LSPs.

Manually configured addresses may hierarchically supercede multiple
level 1 reachable address entries. However, there may be some IP
are not reachable via level 1 routing. If a level 2 router receives
which it is including in its level 2 LSP, but which is not reachable
via level 1 routing in the area, then the packet must be discarded.
In this case, an error report may be returned (as specified in RFC
1009), with the reason for discard specifying destination
unreachable.

Figure 2 - An Example Routing Domain (not shown)

An example is illustrated in figure 2. Suppose that the network
number for the entire routing domain is 17 (a class A network).
Suppose each area is assigned a subnet number consisting of the next
8 bits. The area may be further subdivided by assigning the next
eight bits to each LAN in the area, giving each a 24 bit subnet mask
(counting the network and subnet fields). Finally 8 bits are left for
the host field. Suppose that for a particular area (given subnet
number 17.133) there are a number of IP capable level 1 routers
announcing (in the special IP entry in their level 1 LSPs) subnets
17.133.5, 17.133.43, and 17.133.57.

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Suppose that in this example, in order to save space in level 2 LSPs,
the level 2 routers in this area are configured to announce subnet
17.133. Only this one address needs to be announced in level 2 LSPs.
Thus if an IP packet comes along for an address in subnet 17.133.5,
17.133.43 or 17.133.57, then other level 2 routers, in other areas,
will know to pass the traffic to this area.

The inclusion of 17.133 in level 2 LSPs means that the three subnet
addresses starting with 17.133 do not all have to be listed
separately in level 2 LSPs.

If any traffic comes along that is for an unreachable address such as
17.133.124.7, then level 2 routers in other areas in this particular
domain will think that this area can handle this traffic, will
forward traffic to level 2 routers in this area, which will have to

Suppose that subnet number 17.133.125 was actually reachable via some
other area, such as the lower right hand area. In this case, the
level 2 router in the left area would be announcing (in its level 2
LSPs according to manually configured information) reachability to
subnet 17.133. However, the level 2 router in the lower right area
would be announcing (in its level 2 LSPs according to information
taken from its received level 1 LSPs), reachability to subnet
17.133.125. Due to the use of best match routing, this works
correctly. All traffic from other areas destined to subnet 17.133.125
would be sent to the level 2 router in the lower right area, and all
other traffic to subnet 17.133 (i.e., traffic to any IP address
starting with 17.133, but not starting with 17.133.125) would be sent
to the level 2 router in the leftmost area.

3.3 Addressing Routers in IS-IS Packets

The IS-IS packet formats explicitly require that OSI-style addresses
of routers appear in the IS-IS packets. For example, these addresses
are used to determine area membership of routers. It is therefore
necessary for all routers making use of the IS-IS protocol to have
will be used only in the operation of the IS-IS protocol, and are not
used for any other purpose (such as the operation of EGP, ICMP, or
other TCP/IP protocols).

For OSI-only and dual routers, assignment of NSAP addresses is
straight forward, but is outside of the scope of this specification.
Address assignment mechanisms are being set up by standards bodies
which allow globally unique OSI NSAP addresses to be assigned. All
OSI-only and dual routers may therefore make use of normal OSI
addresses in the operation of the IS-IS protocol.

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For IP-only routers, there are two ways in which NSAP addresses may
be obtained for use with the IS-IS protocol.

1) For those environments in which OSI is being used, or in which it
is anticipated that OSI will be used in the future, it is
permissible to obtain NSAP address assignments in the normal
manner, assign normal NSAP addresses to IP-only routers, and use
these addresses in the operation of IS-IS. This approach is
recommended even for pure IP routing domains, as it will simplify
future migration from IP-only to dual operation.

2) In some cases, routers may have only TCP/IP addresses, and it may
be undesireable to have to go through the normal mechanisms for
provided below for algorithmically generating a valid OSI style
assignments.

Where desired, for IP-only routers, for use in IS-IS packet formats
only, OSI-style addresses (compatible with the USA GOSIP version 2.0
NSAP address format [9]) may be derived as follows:

AFI       1 octet       value "47" (specifies ICD format)

ICD       2 octet       value "00 05" (specifies Internet/Gosip)

DFI       1 octet       value "xx"

AA        3 octets      value "xx xx xx" (specifies special
IP-only use of NSAPs)

Reserved  2 octets      must be "00 00"

RD        2 octets      contains autonomous system number

Area      2 octets      must be assigned as described below

ID        6 octets      must be assigned as described below

SEL       1 octet       used as described below

The AFI value of "47" and the ICD value of "00 05" specifies the
Gosip Version 2.0 addressing format. The DFI number of "xx" and the
AA of "xx xx xx" specify that this special NSAP address format is
being used, solely for IS-IS packet formats in an IP-only
environment. The reserved field must contain "00 00", as specified in
GOSIP version 2.0.

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The routing domain field contains the Autonomous System number.
Strictly speaking, this is not necessary, since the IS-IS packets are
exchanged within a single AS only. However, inclusion of the AS
number in this address format will ensure correct operation in the
event that routers from separate routing domains/ASs are incorrectly
placed on the same link. The AS number in this context is used only
for definition of unique NSAP addresses, and does not imply any
coupling with exterior routing protocols.

The Area field must be assigned by the authority responsible for the
routing domain, such that each area in the routing domain must have a
unique Area value.

The ID must be assigned by the authority responsible for the routing
domain. The ID must be assigned such that every router in the routing
domain has a unique value. It is recommended that one of the
following methods is used:

1)use a unique IEEE 802 48 bit station ID

2)use the value hex "02 00" prepended to an IP address of the router.

IEEE 802 addresses, if used, must appear in IEEE canonical format.

Since the IEEE 802 station IDs are assigned to be globally unique,
use of these values clearly assures uniqueness in the area. Also, all
assigned IEEE 802 station IDs have the global/local bit set to zero.
Prepending the indicated pattern to the front of the IP address
therefore assures that format (2) illustrated above cannot produce
addresses which collide with format (1). Finally, to the extent that
IP addresses are also globally unique, format (2) will produce unique
IDs for routers.

The indicated hex value is specified in IEEE 802 canonical form [10].
In IEEE 802 addresses, the multicast bit is the least significant bit
of the first byte. The global/local bit is the next least significant
bit of the first byte. The indicated prefix therefore sets the
global/local bit to 1, and all other bits in the first two octets to
0.

Note that within an area, whether ISO addresses are configured into
the routers through ISO address assignment, or whether the ISO-style
address is generated directly from the AS number and IP address, all
routers within an area must have the same high order part of address
(AFI, ICD, DFI, AA, RD, and Area). This ISO-style address is used in
IS-IS Hello messages and is the basis by which routers recognize
whether neighbor nodes are in or out of their area.

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External connectivity (i.e., communications with routers outside of
the routing domain) is done only by level 2 routers. The ISO version
of IS-IS allows external OSI routes to be reported as "reachable
address prefixes" in level 2 LSPs. The integrated IS-IS also allows
inter-domain routing) to be reported in level 2 LSPs in the "IP
external reachability information" field. External OSI and external
IP routes are handled independently.

The routes announced in IP external reachability information entries
include all routes to outside of the routing domain. This includes
routes learned from OSPF, EGP, RIP, or any other external protocol.

External routes may make use of "internal" or "external" metrics.
Internal metrics are comparable with the metrics used for internal
routes. Thus in choosing between an internal route, and an external
route using internal metrics, the metric values may be directly
compared. In contrast, external metrics cannot be directly compared
with internal metrics. Any route defined solely using internal
metrics is always preferred to any route defined using external
metrics. When an external route using external metrics must be used,
the lowest value of the external metric is preferred regardless of
the internal cost to reach the appropriate exit point.

It is useful, in the operation of external routing protocols, to
provide a mechanism for border routers (i.e., routers in the same
routing domain, which have the ability to route externally to other
domains) to determine each other's existence, and to exchange
external information (in a form understood only by the border routers
themselves). This is made possible by inclusion of "inter-domain
routing protocol information" fields in level 2 LSPs. The inter-
domain routing protocol information field is not included in
pseudonode LSPs.

In general there may be multiple types of external inter-domain
routing protocol information exchanged between border routers. The
IS-IS therefore specifies that each occurance of the inter-domain
routing protocol information field include a "type" field, which
indicates the type of inter-domain routing protocol information
enclosed. Values to be used in the type field will be specified in
future versions of the "Assigned Numbers" RFC. Initial values for
this field are specified in Annex A of this specification.

Information contained in the inter-domain routing protocol
information field will be carried in level 2 LSPs, and will therefore
need to be stored by all level 2 routers in the domain. However, only

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those level 2 routers which are directly involved in external routing
will use this information. In designing the use of this field, it is
important to carefully consider the implications that this may have
on storage requirements in level 2 routers (including those level 2
routers which are not directly involved in external routing).

The protocols used to exchange routing information directly between
border routers, and external routers (in other routing domains /
autonomous systems) are outside of the scope of this specification.

3.5 Type of Service Routing

The integrated IS-IS protocol provides IP Type of Service (TOS)
routing, through use of the Quality of Service (QOS) feature of IS-
IS. This allows for routing on the basis of throughput (the default
metric), delay, expense, or residual error probability. Note than any
particular packet may be routed on the basis of any one of these four
metrics. Routing on the basis of general combinations of metrics is
not supported.

The support for TOS/QOS is optional. If a particular packet calls for
a specific TOS, and the correct path from the source to destination
is made up of routers all of which support that particular TOS, then
the packet will be routed on the optimal path. However, if there is
no path from the source to destination made up of routers which
support that particular type of service, then the packet will be
forwarded using the default metric instead. This allows for TOS
service in those environments where it is needed, while still
providing acceptable service in the case where an unsupported TOS is
requested.

NOTE - IP does not have a cost TOS. There is therefore no mapping of
IP TOS metrics which corresponds to the minimum cost metric.

The IP TOS field is mapped onto the four available metrics as
follows:

Bits 0-2 (Precedence):  This field does not affect the route, but
rather may affect other aspects of packet
forwarding.

Bits 3 (Delay), 4 (Throughput) and 5 (Reliability):

000     (all normal)            Use default metric

100     (low delay)             Use delay metric

010     (high throughput)       Use default metric

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001     (high reliabiity)       Use reliability metric

other                           Use default metric

3.6 Multiple LSPs and SNPs

In some cases, IS-IS packets (specifically Link State Packets and
Complete Sequence Number Packets) may be too large to fit into one
packet. The OSI IS-IS [1] allows for LSPs and CSNPs to be split into
multiple packets. This is independent of ISO 8473 segmentation, and
is also independent of IP fragmentation. Use of independent multiple
packets has the advantages (with respect to segmentation or
fragmentation) that: (i) when information in the IS-IS changes, only
those packets effected need to be re-issued; (ii) when a single
packet is received, it can be processed without the need to receive
all other packets of the same type from the same router before
beginning processing.

The Integrated IS-IS makes use of the same multiple packet function,
as defined in [1]. IP-specific fields in IS-IS packets may be split
across multiple packets. As specified in section 5 ("Structure and
Encoding of PDUs"), some of the IP-specific fields (those which may
be fairly long) may be split into several occurences of the same
field, thereby allowing splitting of the fields across different
packets.

Multiple LSPs from the same router are distinguished by LSP number.
Generally, most variable length fields may occur in an LSP with any
LSP number. Some specific variable length fields may be required to
occur in LSP number 0. Except where explicitly stated otherwise, when
an IS-IS router issues multiple LSPs, the IP-specific fields may
occur in an LSP with any LSP number.

Complete Sequence Number Packets may be split into multiple packets,
with the range to which each packet applies explicitly reported in
the packet. Partial Sequence Number Packets are inherently partial,
and so can easily be split into multiple packets if this is
necessary. Again, where applicable, IP-specific fields may occur in
any SNP.

3.7 IP-Only Operation

For IP-only routers, the format for IS-IS packets remains unchanged.
However, there are some variable length fields from the IS-IS packets
that can be omitted. Specifically:

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IS-IS Hello Packets:

- no change

- the "End Systems Neighbours" entries are omitted

- the "Prefix Neighbours" entries are omitted

IS-IS Sequence Number Packets:

- no change

3.8 Encapsulation

Future versions of the Integated IS-IS may specify optional
encapsulation mechanisms for partition repair, and for forwarding
packets through incompatible routers (i.e., for forwarding OSI
packets through IP-only routers, and forwarding IP packets through
OSI-only routers). The details of encapsulation and decapsulation are
for further study. Routers complying with the Integrated IS-IS are
not required to implement encapsulation nor decapsulation.

3.9 Authentication

The authentication field allows each IS-IS packet to contain
information used to authenticate the originator and/or contents of
the packet.  The authentication information contained in each packet
is used to authenticate the entire packet, including OSI and IP
parts. If a packet is received which contains invalid authentication
information, then the entire packet is discarded. If an LSP or SNP is
split into multiple packets (as described in section 3.6), then each
is authenticated independently.

Use of the authentication field is optional. Routers are not required
to be able to interpret authentication information. As with other
fields in the integrated IS-IS, if a router does not implement
authentication then it will ignore any authentication field that may
be present in an IS-IS packet.

Annex D specifies a proposed use of the authentication field.

3.10 Order of Preference of Routes / Dijkstra Computation

We define the term "IP reachability entry" to mean the combination of
calculate routes to each distinct IP reachability entry. For the

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Dijkstra calculation, each IP reachability entry can be treated in
much the same manner as an OSI end system. Naturally, each IP
reachability entry is treated as distinct from any OSI end systems
which may also be reachable in the same area or routing domain.

For any particular IP reachability entry, this is the same as another
entry if and only if: (i) the subnet masks are identical; and (ii)
for each bit in the subnet mask which has the value "1", the IP
address is identical. This can easily be tested by zeroing those bits
in the IP address which correspond to a zero bit in the mask, and
then treating the entry as a 64 bit quantity, and testing for
equality between different 64 bit quantities. The actual calculation
of routes to IP reachability entries is therefore no more complex
than calculation of routes to OSI end systems (except for the
replacement of a 48-bit test with a 64-bit test).

The Dijkstra computation does not take into consideration whether a
router is IP-only, OSI-only, or dual. The topological restrictions
specified in section 1.4 ensure that IP packets will only be sent via
IP-capable routers, and OSI packets will only be sent via OSI-capable
routers.

The Integrated IS-IS prefers routes within the area (via level 1
routing) whenever possible. If level 2 routes must be used, then
routes within the routing domain (specifically, those routes using
internal metrics) are prefered to routes outside of the routing
domain (using external metrics).

The Integrated IS-IS protocol makes use of "best match" routing of IP
packets. This implies that a particular destination address may match
more than one entry in the forwarding database. If a particular IP
packet has a destination address which matches two different IP
reachability entries, then the entry who's mask contains the most "1"
bits is preferred.

IP packets whose destination is a router are routed the same way as
any other IP packet, by forwarding first to the appropriate subnet,
and then forwarding on that subnet to the destination host (which
just happens to be a router in this case). In particular, the IP
forwarding database does not contain explicit routes to the
individual "IP interface addresses" listed by each router in its LSP.

However, host routes (routes with a subnet mask of all ones) may of
course be included in the IP reachability entries, and will be
handled in the same manner as other IP reachability entries.

In order to ensure correct interoperation of different router
implementations, it is necessary to specify the order of preference

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of possible routes. For OSI destinations, this is outside of the
scope of this report. For IP destinations, this is specified in
section 3.10.1 and 3.10.2 below. Annex C specifies a detailed
Dijkstra calculation and forwarding algorithm which is compatible
with the order of preference of routes specified here.

With IS-IS, if a route to a given destination is advertised, or a
with some or all of the specified TOS metric types may be associated
with that destination or link. However, the default metric must
always be available. Normally this ensures that if a route using any
TOS metric is available, then a route using the default metric will
also be available. The only exception to this is where the
corresponding route using the default metric has a total cost (within
the area, or within the level 2 backbone) greater than MaxPathMetric.

In determining the route to a particular destination for a specified
TOS, only routes using either the requested TOS metric, or the
default TOS metric, are considered.

3.10.1 Order of Preference of Routes In Level 1 Routing

If a given destination is reachable within an area via a route using
either the requested TOS or the default TOS, then the IS-IS will
always make use of a path within the area (via level 1 routing),
regardless of whether an alternate path exists outside of the area
(via level 2 routing). In this case, routes within the area are
selected as follows:

1) Amongst routes in the area, if the specified destination
then the more specific address match (the one with more "1"
bits in the mask) is prefered.

2) Amongst routes in the area to equally specific address
matches, routes on which the requested TOS (if any) is
supported are always prefered to routes on which the
requested TOS is not supported.

3) Amongst routes in the area of the same TOS to equally
specific address matches, the shortest routes are prefered.
For determination of the shortest path, if a route on which
the specified TOS is supported is available, then the
specified TOS metric is used, otherwise the default metric
is used. Amongst routes of equal cost, load splitting may
be performed as specified in [1].

For a level 1 only router (i.e., a router which does not take part in

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level 2 routing, or a level 2 router which is not "attached"), if a
given destination is not reachable within an area, level 1 routing
will always route to a level 2 router as follows:

1) Amongst routes in the area to attached level 2 routers,
routes on which the requested TOS (if any) is supported
are always prefered to routes on which the requested TOS
is not supported.

2) Amongst routes in the area of the same TOS to attached
level 2 routers, the shortest routes are prefered. For
determination of the shortest path, if a route on which
the specified TOS is supported is available, then the
specified TOS metric is used, otherwise the default
metric is used. Amongst routes of equal cost,
loadsplitting may be performed as specified in [1].

3.10.2 Order of Preference of Routes in Level 2 Routing

For those level 2 routers which also take part in level 1 routing,
routes learned via level 1 routing, using either the requested TOS or
the default TOS, are always prefered to routes learned through level
2 routing. For destinations which are not reachable via level 1
routing, or for level 2 only routers (routers which do not take part
in level 1 routing), then level 2 routes are selected as follows:

1) Routes using internal metrics only are always preferred
to routes using external metrics.

2) If a route using internal metrics only is available:

a) If the specified destination address matches more
specific address match (i.e., the largest number of
"1"s present in the subnet mask) is prefered.

b) Amongst routes with equally specific address matches
(i.e., an equal number of "1"s present in the subnet
mask), routes on which the requested TOS (if any) is
supported are always preferred to routes on which the
requested TOS is not supported.

c) Amongst routes of the same TOS with an equally specific
address matches, the shortest path is prefered. For
determination of the shortest path, if a route on which
the specified TOS is supported is available, then the
specified TOS metric is used, otherwise the default
metric is used. Amongst routes of equal cost,

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loadsplitting may be performed as specified in [1].

NOTE: Internal routes (routes to destinations announced
in the "IP Internal Reachability Information" field),
and external routes using internal metrics (routes to
destinations announced in the "IP External Reachability
Information" field, with a metric of type "internal")
are treated identically for the purpose of the order of
preference of routes, and the Dijkstra calculation.

3) If a route using internal metrics only is not available,
but a route using external metrics is available:

a) If the specified destination address matches more than

NOTE: For external routes, the subnet mask will normally
correspond precisely to the network number. This implies
that this test will always discover equal length matching
strings.  However, this test is included to allow future
migration to more general handling of external addresses.

b) Amongst routes with equally specific matches, routes on
which the requested TOS (if any) is supported are always
preferred to routes on which the requested TOS is not
supported. NOTE: for external routes, the route is
considered to support the requested TOS only if the
internal route to the appropriate border router
supports the requested TOS, and the external route
reported by the border router also supports the
requested TOS.

c) Amongst routes of the same TOS with an equal length
matching address string, the shortest path is prefered.
For determination of the shortest path:

(i)  Routes with a smaller announced external metric
are always prefered.

(ii) Amongst routes with an equal external metric,
routes with a shorter internal metric are prefered.
Amongst routes of equal cost, loadsplitting may be
performed as specified in [1].

For level 2 routers which are announcing manually configured summary
addresses in their level 2 LSPs, in some cases there will exist IP

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not match any addresses which are actually reachable via level 1
routing in the area. Generally, packets to such addresses are handled
according to the following rules:

1) If the specified destination is reachable via level 1 routing,
then according to the order of preference of routes specified
above, the packet will be delivered via level 1 routing.

2) If the specified destination is not reachable via level 1 routing,
but is reachable via 2 routing, and there are other level 2
routers which offer more desireable routes according to the
rules specified above (for example a route with a more specific
match, or a route with an equally specific match which supports
the correct TOS), then level 2 routing will forward the packet
according to the more desireable route.

3) If the specified destination is not reachable via level 1 routing,
router (the router which has received the packet and is trying
to forward it) represents the most desireable route, then the
destination is unreachable and the packet must be discarded.

4 Subnetwork Dependent Functions

Dual routers may receive a combination of OSI packets, and IP
packets. It is necessary for the dual routers to be able to clearly
and unambiguously distinguish the two protocol suites.

This problem is not unique to the integrated IS-IS routing protocol.
In fact, this problem will occur in any multi-protocol environment.
This problem is currently being worked on independently, and is
outside of the scope of this specification.

In general, the link type is a configuration parameter. For example,
whether to use PPP, HDLC, or some other point-to-point protocol over
type, a method must be defined for encapsulation of both OSI and IP
packets. Definition of such methods for common link types is outside
of the scope of this specification.

IP packets are encapsulated directly over the underlying link layer
service, using the normal method for transmssion of IP packets over
each type of link. Similarly OSI packets are encapsulated directly
over the underlying link layer service, using the normal method for
transmission of OSI packets over each type of link. Finally, note
that IS-IS packets are encapsulated using the normal method for

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transmission of OSI packets over any particular link type. This
implies that all IS-IS routers, including IP-only routers, must be
able to receive IS-IS packets using the normal encapsulation for OSI
packets.

4.2 Multiple IP Addresses per Interface

The integrated IS-IS allows each router to have multiple IP addresses
for each physical interface, up to the maximum number which may be
contained in a single "IP Interface Address" field (i.e., up to a
maximum of 63 addresses per interface). For example, where there are
two logical subnets on the same LAN, the interface may have two IP
addresses, one corresponding to each logical subnet. Each IS-IS Hello
packet contains a list of IP addresses associated with the physical
interface over which the Hello is transmitted.

It is permissible to implement routers which conform to the
Integrated IS-IS specification which restrict the number of IP
addresses per interface. However, IP-capable routers must be able to
interact correctly with other routers which assign multiple IP
per interface).

Where appropriate (for example, in some cases on point-to-point
case, the IS-IS Hello transmitted on that interface may omit the IP
field with zero entries.

4.3 LANs, Designated Routers, and Pseudonodes

The maintenance of designated routers and pseudonodes is specified in
[1], and is not changed by this proposal. In the case that IP-only
and dual routers (or OSI-only and dual routers) are mixed on the same
LAN in a pure IP area (or a pure OSI area, respectively), any router
on the LAN may be elected designated router.

However, there is a fundamental difference in the way that OSI and
TCP/IP deal with LANs, and other broadcast subnetworks.

With OSI, the use of the ES-IS protocol (ISO 9542) allows the end
systems and routers to automatically determine their connectivity,
thereby allowing all end systems on the LAN to potentially route via
any of the routers on the LAN.

In contract, TCP/IP explictly assigns subnet identifiers to each
local area network. In some cases, a single physical LAN could have
multiple subnet identifiers assigned to it. In this case, end systems

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(hosts) which have an address on one logical subnet are explicitly
precluded from sending IP packets directly to a router whose address
places it on a different logical subnet. Each router is manually
configured to know which subnets it can reach on each interface. In
the case that there are multiple logical subnets on the same LAN,
each router can only exchange IP packets with those end systems which
are on the same logical subnet. This implies that it is not
sufficient for the pseudonode LSP to announce all subnets on the LAN

It is therefore necessary for each router to announce in its LSPs
those subnets which it can reach on each interface, including
interfaces to broadcast subnetworks such as LANs. The pseudonode LSP
does not specify the IP addresses which are reachable on the LAN
(i.e., does not contain the the IP reachability field).

As specified elsewhere (see the forthcoming update to the
"Requirements of IP Gateways" [4]), routers may send ICMP redirects
only if: (i) the IP packet is being forwarded over the same physical
interface over which it arrived; and (ii) the source address of the
forwarded IP packet, the IP address of this router's interface (as
indicated by the source address of the ICMP redirect), and the IP
address of the router to which the packet is being redirected (again,
as indicated in the ICMP redirect) are all on the same IP subnet.

The IS-IS determines whether an adjacency is to be established
between two routers using means which are independent of the IP
interface addresses of the routers. Where multiple logical subnets
occur on the same physical LAN, this potentially allows adjacencies
to be brought up between two routers which share physical
connectivity to each other, but which don't have a logical subnet in
common. IP-capable IS-IS routers therefore must be able to forward IP
packets over existing adjacencies to routers with which they share
interface of the neighboring router is on a different logical IP
subnet.

For point-to-point links, IS-IS requires exchange of ISO 9542 ISHs,
as the first step in establishing the link between routers. All IS-IS
routers are therefore required to transmit and receive ISO 9542 ISH

The "protocols supported" field (defined in section 5 below) must be
present in all IS-IS Hello packets sent by dual and IP-only routers.
If this field is missing, then it is assumed that the packet was
transmitted by an OSI-only router. Similarly, those 9542 ISHs sent

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over point-to-point links, where there is (or may be) another IS-IS
router at the other end of the point-to-point link, must also
contains the "protocols supported" field. Note that if this field is
mistakenly sent in a 9542 ISH where there is an ordinary OSI-only End
System at the other end of the link, then (in accordance to ISO 9542)
the End System is required to ignore the field and interpret the ISH
correctly. It is therefore safe to always include this field in ISHs

Dual routers must operate in a dual fashion on every link in the
routing domain over which they are running IS-IS. Thus, the value of
the "protocols supported" field must be identical on every link
(i.e., for any one router running IS-IS, all of the Hellos and LSPs
transmitted by it must contain the same "protocols supported"
values).

4.5 Forwarding to Incompatible Routers

There may be times when a dual router has to forward an IP packet to
an OSI-only router, or forward an OSI packet to an IP-only router. In
this case the packet must be discarded. An error report may be
transmitted, in accordance with the IP or ISO 8473 specification
(respectively). The reason for discard specified in the error report
should specify "destination host unreachable" (for IP), or
"destination unreachable" (for OSI).

Similarly, due to errors, in some cases an IP-only router may have to
forward an IP packet to an OSI-only router. Again, the packet must be
discarded, as specified above. This may only occur if IP-only and
OSI-only routers occur in the same area, which is a configuration
error.

5 Structure and Encoding of PDUs

This clause describes the additional packet fields for use of the ISO
IS-IS Intra-Domain Routing protocol in pure IP and dual environments.
Specifically, the same packet types are used as in IS-IS [1], and all
fixed fields remain the same. Additional variable length fields are
defined in this section.

5.1 Overview of IS-IS PDUs

The packets used in IS-IS routing protocol fall into three main
classes: (i) Hello Packets; (ii) Link State Packets (LSPs); and (iii)
Sequence Number Packets (SNPs).

Hello packets are used to initialize and maintain adjacencies between
neighboring routers. There are three types of IS-IS Hello packets:

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(i) "Level 1 LAN IS to IS Hello PDUs" are used by level 1 routers on
broadcast LANs. (ii) "Level 2 LAN IS to IS Hello PDUs" are used by
level 2 routers on broadcast LANs. (iii) "Point-to-Point IS to IS
Hello PDUs" are used on non-broadcast media, such as point-to-point

On point-to-point links, the exchange of ISO 9542 ISHs (intermediate
system Hellos) is used to initialize the link, and to allow each
router to know if there is a router on the other end of the link,
before IS-IS Hellos are exchanged. All routers implementing IS-IS
(whether IP-only, OSI-only, or dual), if they have any interfaces on
point-to-point links, must therefore be able to transmit ISO 9542

information. There are two types of LSPs: (i) "Level 1 Link State
PDUs" are transmitted by level 1 routers. (ii) "Level 2 Link State
PDUs" are transmitted by level 2 routers. Note that level 2 routers
will, in most cases, also be level 1 routers, and will therefore
transmit both sorts of LSPs.

Sequence number PDUs are used to ensure that neighboring routers have
the same notion of what is the most recent LSP from each other
router. The sequence number PDUs therefore serve a similar function
to acknowledgement packets, but allow more efficient operation. There
are four types of sequence number packets: (i) "Level 1 Complete
Sequence Numbers PDU"; (ii) "Level 2 Complete Sequence Numbers PDU";
(iii) "Level 1 Partial Sequence Numbers PDU"; and (iv) "Level 2
Partial Sequence Numbers PDU". A partial sequence number packet lists
the most recent sequence number of one or more LSPs, and operates
much like an acknowlegement. A partial sequence number packet differs
from an conventional acknowledgement in the sense that it may
acknowlege multiple LSPs at once, and in the sense that it may act as
a request for information. A complete sequence number packet contains
the most recent sequence number of all LSPs in the database. A
complete sequence number packet may therefore be used to ensure
synchronization of the database between adjacent routers either
periodically, or when a link first comes up.

5.2 Overview of IP-Specific Information for IS-IS

There are six new fields defined for the Integrated IS-IS: (i)
"Protocols Supported"; (ii) "IP Interface Address"; (iii)
"Authentication Information"; (iv) "IP Internal Reachability
Information"; (v) "IP External Reachability Information"; and (vi)
"Inter-Domain Routing Protocol Information".

The "Protocols Supported" field identifies the protocols which are

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supported by each router. This field must be included in all IS-IS
Hello packets and all LSPs with LSP number 0 transmitted by IP-
capable routers. If this field is not included in an IS-IS Hello
packet or an LSP with LSP number 0, it may be assumed that the packet
was transmitted by an OSI-only router. The "Protocols Supported"
field must also be included in ISO 9542 ISHs send by IP-capable
routers over point-to-point links to other IS-IS routers.

The "IP Interface Address" is included in all IS-IS Hello packets and
LSPs transmitted by IP-only and dual routers. In the Hello packets,
this field occurs once only, and contains the IP address(es) of the
interface on which the Hello packet is transmitted (up to a maximum
of 63 IP addresses on each interface). If an IS-IS Hello is
transmitted over an interface which does not have an IP address
assigned, then this field may be omitted, or may be included with
zero entries. In Link State Packets, this field contains a list of
one or more IP addresses corresponding to one or more interfaces of
the router which originates the LSP. Each IP-capable router must
include this field in its LSPs. This field may occur multiple times
in an LSP, and may occur in an LSP with any LSP number.

The "Authentication Information" field is optional in all IS-IS PDUs.
If used, it contains information used to authenticate the packet. All
IS-IS packets (including 9542 IS Hellos) may be authenticated by use
of this field.

The "IP Internal Reachability Information" field may be present in
all LSPs transmitted by IP-capable routers. If present, it identifies
a list of zero or more [IP address, subnet mask, metrics] reachable
by the router which originates the LSP. Each entry must contain a
default metric, and may contain delay, expense, and error metrics. If
an IP-capable router does not directly reach any IP addresses, then
it may omit this field, or may include the field with zero [IP
this field includes only entries directly reachable by the router
which originates the LSP, via one of its interfaces. If included in
level 2 LSPs, this field includes only entries reachable by the
router which originates the LSP, either via one of its interfaces, or
indirectly via level 1 routing. This field may occur multiple times
in an LSP, and may occur in an LSP with any LSP number.

The "IP External Reachability Information" field may be present in
level 2 LSPs transmitted by level 2 IP-capable routers. If present,
it identifies a list of zero or more [IP address, subnet mask,
metrics] entries reachable by the router which originates the level 2
LSP. Each entry must contain a default metric, and may contain delay,
expense, and error metrics. Each entry may contain metrics of type
"internal", or of type "external". If a level 2 router does not have

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any external routes (via neighboring routers in other routing
domains), when it may omit this field, or may include the field with
zero entries. This field includes only entries reachable by the
router which originates the LSP, via a direct link to an external
router. This field may occur multiple times in a level 2 LSP, and may
occur in an LSP with any LSP number.

The "Inter-Domain Routing Protocol Information" field may be present
in level 2 LSPs transmitted by level 2 IP-capable routers. This field
is transmitted for the convenience of the external routing protocol,
and is not used by the IS-IS. For example, this may be used to allow
border routers to find each other. This field may occur multiple
times in a level 2 LSP, and may occur in an LSP with any LSP number.

The DP 10589 version of the OSI IS-IS does not currently allow
addition of TLV-encoded variable length fields to Sequence Number
Packets. However, this is being corrected in future versions of
10589. In addition, this is expected to be the only correction to
future versions of 10589 that is not backward-compatible with the DP
version. The Integrated IS-IS therefore makes use of a corrected
version of DP 10589, such that the encoding of SNPs has been fixed.
The correct encoding of sequence number packets (as is expected to
appear in future versions of ISO 10589) is given in Annex B of this
specification.

All IP-specific information is encoded in IS-IS packets as variable
length fields. All variable length fields in IS-IS are encoded as
follows:

No. of Octets
+---------------------------+
|           CODE            |      1
+---------------------------+
|          LENGTH           |      1
+---------------------------+
|           VALUE           |      LENGTH
+---------------------------+

Figure 3 - Encoding of Variable Length Fields

Any codes in a received PDU that are not recognised shall be ignored
and, for those packets which are forwarded (specifically Link State
Packets), passed on unchanged.

In general, an IS-IS PDU may contain multiple variable length fields,
some of which contain OSI-specific information (specified in [1]) and
some of which contain IP-specific information (specified below).
Except where explicitly stated otherwise, these variable length

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fields may occur in any order.

5.3 Encoding of IP-Specific Fields in IS-IS PDUs

This section specifies the detailed encoding of all IP-specific
fields in IS-IS PDUs. Where a particular field may be present in more
than one type of PDU, the field is repeated for each type of PDU to
which it applies.

Bit and octet numbering is the same as in [1]. In particular, octets
in a PDU are numbered starting from 1, in increasing order. Bits in
an octet are numbered from 1 to 8, where bit 1 is the least
significant bit and is pictured on the right. When consecutive octets
are used to represent a number, the lower octet number has the most
significant value.

5.3.1 Level 1 LAN IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying

x CODE - 129

x LENGTH - total length of the value field (one octet per
protocol supported).

x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
each supported data protocol.
No. of Octets
+---------------------------+
|           NLPID           |       1
+---------------------------+
:                           :
:                           :
|---------------------------|
|           NLPID           |       1
+---------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

corresponding to the SNPA over which this PDU is to be transmitted.

x CODE - 132

x LENGTH - total length of the value field (four octets per address).

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x VALUE -
No. of Octets
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
+----------------------------+

7 Authentication Information -- Information used to authenticate the
PDU

x CODE - 133

x LENGTH - total length of the value field.

x VALUE - TBD.

5.3.2 Level 2 LAN IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying

x CODE - 129

x LENGTH  - total length of the value field (one octet per protocol
supported).

x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
supported data protocol.
No. of Octets
+----------------------------+
|           NLPID            |      1
+----------------------------+
:                            :
:                            :
+----------------------------+
|           NLPID            |      1
+----------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

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corresponding to the SNPA over which this PDU is to be transmitted.

x CODE - 132

x LENGTH - total length of the value field (four octets per address).

x VALUE -
No. of Octets
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
+----------------------------+

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

5.3.3 Point-to-Point IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying

x CODE - 129

x LENGTH - total length of the value field (one octet per protocol
supported).

x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
supported data protocol.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+----------------------------+
|           NLPID            |      1
+----------------------------+
:                            :
:                            :
+----------------------------+
|           NLPID            |      1
+----------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

corresponding to the SNPA over which this PDU is to be transmitted.

x CODE - 132

x LENGTH - total length of the value field (four octets per address).

x VALUE -
No. of Octets
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
+----------------------------+

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

5.3.4 Level 1 Link State PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying.

This must appear once in LSP number 0.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

x CODE - 129

x LENGTH - total length of the value field (one octet per protocol
supported).

x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
supported data protocol.
No. of Octets
+----------------------------+
|           NLPID            |      1
+----------------------------+
:                            :
:                            :
+----------------------------+
|           NLPID            |      1
+----------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

7 IP Interface Addresses -- The IP addresss of one or more interfaces
corresponding to the SNPAs enabled on this Intermediate system
(i.e., one or more IP addresses of this router).

This is permitted to appear multiple times, and in an LSP with
any LSP number.

x CODE - 132

x LENGTH - total length of the value field (four octets per address).

x VALUE -
No. of Octets
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
+----------------------------+

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

x VALUE - TBD

7 IP Internal Reachability Information -- IP addresses within the
routing domain reachable directly via one or more interfaces on
this Intermediate system.

This is permitted to appear multiple times, and in an LSP with any
LSP number. However, this field must not appear in pseudonode LSPs.

x CODE - 128.

x LENGTH - a multiple of 12.

x VALUE -
No. of Octets
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+

DEFAULT METRIC is the value of the default metric for the link
to the listed neighbor. Bit 8 of this field is reserved, and
must be set to zero on tranmission and ignored on reception.
Bit 7 of this field (marked I/E) indicates the metric type

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(internal or external) for all four TOS metrics, and must be
set to zero indicating internal metrics.

DELAY METRIC is the value of the delay metric for the link to the
listed neighbor. If this IS does not support this metric it shall
set the bit "S" to 1 to indicate that the metric is unsupported.
Bit 7 of this field is reserved, and must be set to zero on
transmission and ignored on reception.

EXPENSE METRIC is the value of the expense metric for the link to
the listed neighbor. If this IS does not support this metric it
shall set the bit "S" to 1 to indicate that the metric is
unsupported. Bit 7 of this field is reserved, and must be set to
zero on transmission and ignored on reception.

ERROR METRIC is the value of the error metric for the link to
the listed neighbor. If this IS does not support this metric it
shall set the bit "S" to 1 to indicate that the metric is
unsupported. Bit 7 of this field is reserved, and must be set
to zero on transmission and ignored on reception.

5.3.5 Level 2 Link State PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying.

This must appear once in LSP number 0.

x CODE - 129

x LENGTH - total length of the value field (one octet per
protocol supported).

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x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
each supported data protocol.
No. of Octets
+----------------------------+
|           NLPID            |      1
+----------------------------+
:                            :
:                            :
+----------------------------+
|           NLPID            |      1
+----------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

7 IP Interface Addresses -- The IP addresss of one or more interfaces
corresponding to the SNPAs enabled on this Intermediate system
(i.e., one or more IP addresses of this router).

This is permitted to appear multiple times, and in an LSP with
any LSP number. Where a router is both a level 1 and level 2 router,
it must include the same IP addresses in its level 1 and level 2 LSPs.

x CODE - 132

x LENGTH - total length of the value field (four octets per address).

x VALUE-
No. of Octets
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
+----------------------------+

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

7 IP Internal Reachability Information -- IP addresses within the
routing domain reachable directly via one or more interfaces on

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this Intermediate system.

This is permitted to appear multiple times, and in an LSP with
any LSP number. However, this field must not appear in pseudonode
LSPs.

x CODE - 128.

x LENGTH -  a multiple of 12.

x VALUE -
No. of Octets
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+

DEFAULT METRIC is the value of the default metric for the link
to the listed neighbor. Bit 8 of this field is reserved, and must
be set to zero on transmission and ignored on reception. Bit 7
of this field indicates the metric type (internal or external)
for all four TOS metrics, and must be set to zero indicating
internal metrics.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

DELAY METRIC is the value of the delay metric for the link to
the listed neighbor. If this IS does not support this metric it
shall set the bit "S" to 1 to indicate that the metric is
unsupported. Bit 7 of this field is reserved, and must be set
to zero on transmission and ignored on reception.

EXPENSE METRIC is the value of the expense metric for the link to
the listed neighbor. If this IS does not support this metric it
shall set the bit "S" to 1 to indicate that the metric is
unsupported. Bit 7 of this field is reserved, and must be set
to zero on transmission and ignored on reception.

ERROR METRIC is the value of the error metric for the link to the
listed neighbor. If this IS does not support this metric it shall
set the bit "S" to 1 to indicate that the metric is unsupported.
Bit 7 of this field is reserved, and must be set to zero on
transmission and ignored on reception.

7 IP External Reachability Information -- IP addresses outside the
routing domain reachable via interfaces on this Intermediate
system.

This is permitted to appear multiple times, and in an LSP with
any LSP number. However, this field must not appear in pseudonode LSPs.

x CODE - 130.

x LENGTH - a multiple of 12.

x VALUE -

Callon                                                         [Page 46]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+
:                            :
:                            :
+----------------------------+
| 0 |I/E|   DEFAULT METRIC   |      1
+----------------------------+
| S | R |    DELAY METRIC    |      1
+----------------------------+
| S | R |   EXPENSE METRIC   |      1
+----------------------------+
| S | R |    ERROR METRIC    |      1
+----------------------------+
+----------------------------+
+----------------------------+

DEFAULT METRIC is the value of the default metric for the
path to the listed IP addresses. Bit 8 of this field is
reserved, and must be set to zero on transmission and ignored
on reception.  Bit 7 of this field indicates the metric type
(internal or external) for all four TOS metrics, and may be
set to zero indicating internal metrics, or may be set to 1
indicating external metrics.

DELAY METRIC is the value of the delay metric for the path
to the listed IP addresses. If this IS does not support this
metric it shall set the bit "S" to 1 to indicate that the metric
is unsupported. Bit 7 of this field is reserved, and must be
set to zero on transmission and ignored on reception.

EXPENSE METRIC is the value of the expense metric for the link
to the listed IP addresses. If this IS does not support this
metric it shall set the bit "S" to 1 to indicate that the metric
is unsupported.  Bit 7 of this field is reserved, and must be

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

set to zero on transmission and ignored on reception.

ERROR METRIC is the value of the error metric for the link to
the listed IP addresses. If this IS does not support this metric
it shall set the bit "S" to 1 to indicate that the metric is
unsupported. Bit 7 of this field is reserved, and must be set to
zero on transmission and ignored on reception.

7 Inter-Domain Routing Protocol Information -- Inter-domain routing
protocol information carried transparently through level 2 for
the convenience of any Inter-Domain protocol that may be running
in the boundary ISs.

This is permitted to appear multiple times, and in an LSP with
any LSP number.

x CODE - 131.

x LENGTH - total length of the value field

x VALUE -
No. of Octets
+-------------------------------+
| Inter-Domain Information Type |      1
+-------------------------------+
|     External Information      |      VARIABLE
+-------------------------------+

INTER-DOMAIN INFORMATION TYPE indicates the type of the
external information which is encoded in the field.

EXTERNAL INFORMATION contains inter-domain routing protocol
information, and is passed transparently by the IS-IS protocol.

5.3.6 Level 1 Complete Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

x VALUE - TBD

5.3.7 Level 2 Complete Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

5.3.8 Level 1 Partial Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

5.3.9 Level 2 Partial Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

5.3.10 ISO 9542 ISH PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
for Network Layer protocols that this Intermediate System is
capable of relaying.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

This appears in ISO 9542 ISH PDUs transmitted on point-to-point

x CODE - 129

x LENGTH - total length of the value field (one octet per
protocol supported).

x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
each supported data protocol.
No. of Octets
+----------------------------+
|           NLPID            |      1
+----------------------------+
:                            :
:                            :
+----------------------------+
|           NLPID            |      1
+----------------------------+
NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE - TBD

6 Security Considerations

The integrated IS-IS has a provision for carrying authentication
information in all IS-IS packets. This is extensible to multiple
authentication mechanisms. However, currently the only defined
mechanism is a simple password, transmitted in the clear without
encryption (see Annex D). The use of a simple password does not
provide useful protection against intentional misbehavior. Rather,
this should be thought of as a weak protection against accidental
errors such as accidental mis-configuration. Definition of other
authentication mechanisms is beyond the scope of this document.

Other aspects of security are not discussed in this document.

Callon                                                         [Page 50]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

Ross Callon
Digital Equipment Corporation
550 King Street, LKG 1-2/A19
Littleton, MA 01460-1289
508-486-5009

8 References

[1]     "Intermediate System to Intermediate System Intra-Domain
Routeing Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-mode Network Service
(ISO 8473)", ISO DP 10589, February 1990.

[2]     "Protocol for Providing the Connectionless-Mode Network
Service", ISO 8473, March 1987.

[3]     "End System to Intermediate System Routeing Exchange Protocol
for Use in Conjunction with the Protocol for Providing the
Connectionless-Mode Network Service (ISO 8473)", ISO 9542,
March 1988.

[4]     Braden,R., and Postel,J., "Requirements for Internet Gateways",
RFC 1009, June 1987.

[5]     Moy,J., "The OSPF Specification", RFC 1131, October 1989.

[6]     Postel,J., "Internetwork Protocol", RFC 791, September 1981.

[7]     Postel,J., "Internet Control Message Protocol", RFC 792,
September 1981.

[8]     "MIB for Use with the Extended OSI IS-IS in TCP/IP and Dual
Environments", forthcoming.

[9]     GOSIP Advanced Requirements Group, "Government Open Systems
Interconnection Profile (GOSIP) Version 2.0 [Final Text]",
Federal Information Processing Standard, U.S. Department of
Commerce, National Institute of Standards and Technology,
Gaithersburg, MD, October 1990.

[10]    "Standard for Local Area Networks and Metropolitan Area
Networks: Overview and Architecture of Network Standards",
IEEE Standard 802.1a-1990.

Callon                                                         [Page 51]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

Annex A
Inter-Domain Routing Protocol Information

This annex specifies the contents and encoding of the Inter-Domain
Routing Protocol Information (IDRPI) field. This annex is an integral
part of the Integrated IS-IS specification. However, it is expected
that this annex may be augmented or superceded by future efforts
outside of the scope of the IS-IS specification.

A.1 Inter-Domain Information Type

As specified in sections 3.4 and 5.3, the IDRPI field consists of a
one-octet inter-domain information type field, plus a variable
external information field. This section specifies initial values for
the inter-domain information type field.  Other values for inter-
domain information type will be assigned and maintained in future
versions of the "Assigned Numbers" RFC.

The following types have been assigned:

Type = 0        reserved

Type = 1        local (uses routing-domain specific format)

Type = 2        AS Number Tag

Type = 1 indicates that the inter-domain routing protocol information
uses a format which is local to the routing domain.

Type = 2 indicates that the inter-domain routing protocol information
includes autonomous system information used to tag IP external
reachability information. In this case the inter-domain routing
protocol information entry must include a single AS number, which is
used to tag all subsequent External IP Reachability entries until the
end of the LSP, or until the next occurence of the Inter-Domain
Routing Protocol Information field.

A.2 Encoding

As specified in section 5.3.5, the IDPRI entry is encoded as a
variable length field, as follows:

x CODE - 131

x LENGTH - total length of the value field

x VALUE -

Callon                                                         [Page 52]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+-------------------------------+
| Inter-Domain Information Type |      1
+-------------------------------+
|     External Information      |      VARIABLE
+-------------------------------+

INTER-DOMAIN INFORMATION TYPE indicates the type of the
external information which is encoded in the field.

EXTERNAL INFORMATION contains inter-domain routing protocol
information, and is passed transparently by the IS-IS protocol.

The Inter-domain information type field indicates the type of
information which is contained in the external information field, as
follow:

Type = 0 is reserved (must not be sent, and must be ignored on receipt).

Type = 1 indicates that the external information field contains
information which follows a locally specified format.

Type = 2 indicates that the external information field contains an
autonomous system number tag, to be applied to subsequent IP external
reachability information entries. In this case, this "inter-domain
routing protocol information" entry must contain precisely one 2
octet AS number. The AS tag is associated with subsequent IP External
Reachability entries, until the end of the LSP, or until the next
occurence of the Inter-Domain Routing Protocol Information field.
In this case, the VALUE contains the following:

x VALUE -
No. of Octets
+---------------------------------+
| Inter-Domain Information Type=2 |      1
+---------------------------------+
|   Autonomous System Number      |      2
+---------------------------------+

Callon                                                         [Page 53]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

Annex B
Encoding of Sequence Number Packets

The Integrated IS-IS protocol defined in this specification makes use
of the ISO Draft Proposed standard for Intra-domain routing (ISO DP
10589 [1]) as the base routing protocol, upon which IP support may be

However, DP 10589 contains a bug regarding encoding of the variable
length fields in Sequence Number Packets. In particular, DP 10589
encodes the variable length fields in SNPs in a manner which is not
flexible (additional variable length fields cannot be defined for
sequence number packets), and which is inconsistent with the encoding
of the variable length fields in all other IS-IS and ES-IS packets.

The encoding of the variable length fields in SNPs is expected to be
fixed in future versions of 10589. Also, this bug represents the only
expected change to 10589 which cannot be made backward compatible
with existing DP 10589 implementations. For these reasons, the
current version of the Integrated IS-IS will use the anticipated
future encoding of the variable length part of the SNPs. This should
allow future versions of this specification to be compatible with
implementations based on this specification.

This annex specifies the encoding of SNPs, as amended to fix the
encoding of variable length fields. This annex is an integral part of
the Integrated IS-IS specification.

The encoding of SNPs for OSI-only use is shown in this section. For
IP-only or Integrated use, the additional variable length fields
specified in sections 5.3.6 through 5.3.9 are also applicable to
SNPs.

Callon                                                         [Page 54]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

B.1 Level 1 Complete Sequence Numbers PDU

No. of Octets
+--------------------------------+
|     INTRA-DOMAIN ROUTEING      |      1
|     PROTOCOL DISCRIMINATOR     |
+--------------------------------+
|        LENGTH INDICATOR        |      1
+--------------------------------+
|    VERSION/PROTOCOL ID EXT     |      1
+--------------------------------+
|            RESERVED            |      1
+--------------------------------+
| R | R | R |        TYPE        |      1
+--------------------------------+
|            VERSION             |      1
+--------------------------------+
|              ECO               |      1
+--------------------------------+
|            USER ECO            |      1
+--------------------------------+
|           PDU LENGTH           |      2
+--------------------------------+
|           SOURCE ID            |      7
+--------------------------------+
|          START LSP ID          |      8
+--------------------------------+
|           END LSP ID           |      8
+================================+====================
|     VARIABLE LENGTH FIELDS     |      VARIABLE
+--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 24. Note bits 6, 7 and 8 are Reserved,
which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

Callon                                                         [Page 55]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)
generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this
Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this
Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

No. of Octets
+--------------------------------+
|              CODE              |      1
+--------------------------------+
|             LENGTH             |      1
+--------------------------------+
|             VALUE              |      LENGTH
+--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- This may appear multiple times. The option fields,
if they appear more than once, shall appear sorted into ascending
LSPID order.

x CODE - 9

x LENGTH - total length of the value field.

x VALUE - a list of LSP entries of the form:

Callon                                                         [Page 56]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+
:                                :
:                                :
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).

Callon                                                         [Page 57]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

B.2 Level 2 Complete Sequence Numbers PDU

No. of Octets
+--------------------------------+
|     INTRA-DOMAIN ROUTEING      |      1
|     PROTOCOL DISCRIMINATOR     |
+--------------------------------+
|        LENGTH INDICATOR        |      1
+--------------------------------+
|    VERSION/PROTOCOL ID EXT     |      1
+--------------------------------+
|            RESERVED            |      1
+--------------------------------+
| R | R | R |        TYPE        |      1
+--------------------------------+
|            VERSION             |      1
+--------------------------------+
|              ECO               |      1
+--------------------------------+
|            USER ECO            |      1
+--------------------------------+
|           PDU LENGTH           |      2
+--------------------------------+
|           SOURCE ID            |      7
+--------------------------------+
|          START LSP ID          |      8
+--------------------------------+
|           END LSP ID           |      8
+================================+====================
|     VARIABLE LENGTH FIELDS     |      VARIABLE
+--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 25. Note bits 6, 7 and 8 are Reserved,
which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

Callon                                                         [Page 58]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)
generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this
Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this
Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

No. of Octets
+--------------------------------+
|              CODE              |      1
+--------------------------------+
|             LENGTH             |      1
+--------------------------------+
|             VALUE              |      LENGTH
+--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- this may appear multiple times. The option fields,
if they appear more than once, shall appear sorted into ascending
LSPID order.

x CODE - 9

x LENGTH - total length of the value field.

x VALUE - a list of LSP entries of the form:

Callon                                                         [Page 59]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+
:                                :
:                                :
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).

Callon                                                         [Page 60]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

B.3 Level 1 Partial Sequence Numbers PDU

No. of Octets
+--------------------------------+
|     INTRA-DOMAIN ROUTEING      |      1
|     PROTOCOL DISCRIMINATOR     |
+--------------------------------+
|        LENGTH INDICATOR        |      1
+--------------------------------+
|    VERSION/PROTOCOL ID EXT     |      1
+--------------------------------+
|            RESERVED            |      1
+--------------------------------+
| R | R | R |        TYPE        |      1
+--------------------------------+
|            VERSION             |      1
+--------------------------------+
|              ECO               |      1
+--------------------------------+
|            USER ECO            |      1
+--------------------------------+
|           PDU LENGTH           |      2
+--------------------------------+
|           SOURCE ID            |      7
+================================+====================
|     VARIABLE LENGTH FIELDS     |      VARIABLE
+--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5)  26. Note bits 6, 7 and 8 are Reserved,
which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)

Callon                                                         [Page 61]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

No. of Octets
+--------------------------------+
|              CODE              |      1
+--------------------------------+
|             LENGTH             |      1
+--------------------------------+
|             VALUE              |      LENGTH
+--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7  LSP Entries - this may appear multiple times. The option fields,
if they appear more than once, shall appear sorted into ascending
LSPID order.

x CODE - 9

x LENGTH - total length of the value field.

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

x VALUE - a list of LSP entries of the form:

No. of Octets
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+
:                                :
:                                :
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number
octet of the LSPID is the least significant octet).

Callon                                                         [Page 63]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

B.4 Level 2 Partial Sequence Numbers PDU

No. of Octets
+--------------------------------+
|     INTRA-DOMAIN ROUTEING      |      1
|     PROTOCOL DISCRIMINATOR     |
+--------------------------------+
|        LENGTH INDICATOR        |      1
+--------------------------------+
|    VERSION/PROTOCOL ID EXT     |      1
+--------------------------------+
|            RESERVED            |      1
+--------------------------------+
| R | R | R |        TYPE        |      1
+--------------------------------+
|            VERSION             |      1
+--------------------------------+
|              ECO               |      1
+--------------------------------+
|            USER ECO            |      1
+--------------------------------+
|           PDU LENGTH           |      2
+--------------------------------+
|           SOURCE ID            |      7
+================================+====================
|    VARIABLE LENGTH FIELDS      |      VARIABLE
+--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 27. Note bits 6, 7 and 8 are Reserved,
which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)

Callon                                                         [Page 64]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

No. of Octets
+--------------------------------+
|              CODE              |      1
+--------------------------------+
|             LENGTH             |      1
+--------------------------------+
|             VALUE              |      LENGTH
+--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- this may appear multiple times. The option fields,
if they appear more than once, shall appear sorted into ascending
LSPID order.

x CODE - 9

x LENGTH - total length of the value field.

x VALUE - a list of LSP entries of the form:

Callon                                                         [Page 65]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

No. of Octets
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+
:                                :
:                                :
+--------------------------------+
+--------------------------------+
|             LSP ID             |      8
+--------------------------------+
|         LSP SEQ NUMBER         |      4
+--------------------------------+
|            CHECKSUM            |      2
+--------------------------------+

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER  -Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).

Callon                                                         [Page 66]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

Annex C
Dijkstra Calculation and Forwarding

Annex C.2 of ISO DP 10589 [1] specifies the SPF (Dikskstra) algorithm
for calculating routes with the IS-IS routing protocol. This annex
specifies modifications to the SPF algorithm for supporting IP and
dual routing, and specifies a compatible method for forwarding IP
packets. This will result in an order of preference of routes which
is compatible with that specified in section 3.10.

This annex is included for informational purposes.

C.1 SPF Algorithm for IP and Dual Use

This section specifies an SPF Algorithm for calculating routes with
the IS-IS routing protocol, for support of both TCP/IP and OSI. This
is based on an extention to the algorithm specified in annex C.2 of
ISO DP 10589 [1].

An algorithm invented by Dijkstra known as shortest path first (SPF)
is used as the basis for the route calculation. It has a
computational complexity of the square of the number of nodes, which
can be decreased to the number of links in the domain times the log
of the number of nodes for sparse networks (networks which are not
highly connected).

A number of additional optimizations are possible:

1) If the routing metric is defined over a small finite field (as in
this standard), the factor of log n may be removed by using data
structures which maintain a separate list of systems for each value
of the metric rather than sorting the systems by logical distance.

2) Updates can be performed incrementally without requiring a complete
recalculation. However, a full update must be done periodically to
ensure recovery from data corruption, and studies suggest that with
a very small number of link changes (perhaps 2) the expected
computation complexity of the incremental update exceeds the
complete recalculation. Thus, this annex specifies the algorithm
only for the full update.

3) If only End System LSP information has changed, it is not necessary
to re-compute the entire Dijkstra tree. If the proper data
structures are used, End Systems (including IP reachability
entries) may be attached and detached as leaves of the tree and
their forwarding information base entries altered as appropriate.

The original SPF algorithm does not support load splitting over

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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

multiple paths. The algorithm in this annex does permit load
splitting by identifying a set of equal cost paths to each
destination rather than a single least cost path.

C.1.1 Databases

PATHS -- This represents an acyclic directed graph of shortest paths
from the system S performing the calculation. It is stored as a set
of triples of the form <N,d(N),{Adj(N)}>, where:

N is a system identifier. In the level 1 algorithm, N is a
6 octet ID for OSI end systems, a 7 octet ID for routers, or
an 8 octet IP Internal Reachability Information entry. For a
router which is not a pseudonode, it is the 6 octet system ID,
with a 0 appended octet. For a pseudonode it is a true 7 octet
quantity, comprised of the 6 octet Designated Intermediate
System ID and the extra octet assigned by the Destinated Router.
The IP Internal Reachability Information entries consist of a
4 octet IP address plus a 4 octet subnet mask, and will always
be a leaf, i.e., "End System" in PATHS.

In the level 2 algorithm, N is either a 7 octet router or
pseudonode ID (as in the level 1 algorithm); a variable
length OSI address prefix; an 8 octet IP Internal Reachability
Information Entry, or an 8 octet IP External Reachability
Information entry. The variable length OSI address prefixes,
and 8 octet IP Reachability Information entries will always
be a leaf, i.e., "End System" in PATHS. As above, the IP
Reachability Information entries consist of an [IP address,

d(N) is N's distance from S (i.e., the total metric value
from N to S).

{Adj(N)} is a set of valid adjacencies that S may use for
forwarding to N.

When a system is placed on PATHS, the path(s) designated by its
position in the graph is guaranteed to be a shortest path.

TENT -- This is a list of triples of the form <N,d(N),{Adj(N)}>,
where N, d(N), and {Adj(N)} are as defined above for PATHS.

TENT can intuitively be thought of as a tentative placement
of a system in PATHS. In other words, the triple <N,x,{A}>
in TENT means that if N were placed in PATHS, d(N) would be x,
but N cannot be placed on PATHS until is is guaranteed that
no path shorter than x exists.

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Similarly, the triple <N,x,{A,B}> in TENT means that if N
were placed in PATHS, then d(N) would be x via either

Note: It is suggested that the implementation maintain the database
TENT as a set of list of triples of the form <*,Dist,*>, sorted by
distance Dist. In addition, it is necessary to be able to process
those systems which are pseudonodes before any non-pseudonodes at the
same distance Dist.

The 8 octet system identifiers which specify IP reachability entries
must always be distinguishable from other system identifiers. As
specified in section 3.10, two IP reachability entries which differ
only in the subnet mask are still considered to be separate, and will
therefore have distinct system identifiers N. The SPF algorithm will
therefore calculate routes to each such entry, and the correct entry
will be selected in the forwarding process.

C.1.2 Use of Metrics in the SPF Algorithm

Internal metrics are not comparable to external metrics. For external
routes (routes to destinations outside of the routing domain), the
cost d(N) of the path from N to S may include both internal and
external metrics. d(N) may therefore be maintained as a two-
dimensioned vector quantity (specifying internal and external metric
values).

d(N) is initialized to [internal metric = 0, external metric = 0].

In incrementing d(N) by 1, if the internal metric value is less than
the maximum value MaxPathMetric, then the internal metric value is
incremented by one and the external metric value left unchanged; if
the internal metric value is equal to the maximum value
MaxPathMetric, then the internal metric value is set to 0 and the
external metric value is incremented by 1. Note that this can be
implemented in a straightforward manner by maintaining the external
metric as the high order bits of the distance.

In the code of the algorithm below, the current path length is held
in the variable "tentlength". This variable is a two-dimensional
quantity tentlength=[internal metric, external metric], and is used
for comparing the current path length with d(N) as described above.
Tentlength is incremented in the same manner as d(N).

C.1.3 Overview of the Algorithm

The basic algorithm, which builds PATHS from scratch, starts out by
putting the system doing the computation on PATHS (no shorter path to

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SELF can possibly exist). TENT is then pre-loaded from the local

Note that a system is not placed on PATHS unless no shorter path to
that system exists. When a system N is placed on PATHS, the path to
each neighbor M of N, through N, is examined, as the path to N plus
the link from N to M. If <M,*,*> is in PATHS, this new path will be
longer, and thus ignored.

If <M,*,*> is in TENT, and the new path is shorter, the old entry is
removed from TENT and the new path is placed in TENT. If the new path
is the same length as the one in TENT, then the set of potential
adjacencies {Adj(M)} is set to the union of the old set (in TENT) and
the new set {Adj(N)}. If M is not in TENT, then the path is added to
TENT.

Next the algorithm finds the triple <N,x,{Adj(N)}> in TENT, with
minimal x. Note: This is done efficiently because of the optimization
described above. When the list of triples for distance Dist is
exhausted, the algorithm then increments Dist until it finds a list
with a triple of the form <*,Dist,*>.

N is placed in PATHS. We know that no path to N can be shorter than x
at this point because all paths through systems already in PATHS have
already been considered, and paths through systems in TENT still have
to be greater than x because x is minimal in TENT.

When TENT is empty, PATHS is complete.

Note that external metrics can only occur in "IP External
Reachability Information" entries, which correspond to a leaf (i.e.,
End System in PATHS). Any route utilizing an entry with an external
metric will always be considered to be less desireable than any entry
which uses an internal metric. This implies that in the addition of
systems to PATHS, all systems reachable via internal routes are
always added before any system reachable via external routes.

C.1.4 The Algorithm

The Decision Process Algorithm must be run once for each supported
routing metric (i.e., for each supported Type of Service). A level 1
router runs the algorithm using the level 1 LSP database to compute
level 1 paths (for those level 1 routers which are not level 2
routers, this includes the path to the nearest attached level 2
router). Level 2 routers also separately run the algorithm using the
level 2 LSP database to compute level 2 paths. IP-capable level 2
routers must keep level 2 internal IP routes separate from level 2
external IP routes.

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Note that this implies that routers which are both level 1 and level
2 routers, and which support all four routing metrics, must run the
SPF algorithm 8 times (assuming partition repair is not implemented).

If this system is a Level 2 Router which supports the partition
repair optional function the Decision Process algorithm for computing
Level 1 paths must be run twice for the default metric. This first
execution is done to determine which of the area's
manualAreaAddresses are reachable in this partition, and to elect a
Partition Designated Level 2 Router for the partition. The partition
Designated Level 2 Router will determine if the area is partitioned
and will create virtual Level 1 links to the other Partition
Designated Level 2 Routers in the area in order to repair the Level 1
partition. This is further described in section 7.2.10 of [1].

The SPF algorithm specified here will calculate routes for both OSI
and IP. In particular, routes are calculated to all system
identifiers N, where N may specify an OSI End System, the OSI address
of a router, or an IP reachability entry. In computing the forwarding
database, it is an implementation specific issue whether the IP
forwarding database is kept separately from the OSI forwarding
database. Where appropriate, this annex will refer separately to
entries in these two forwarding data bases. This is not meant to
preclude any specific implementation method.

OSI and IP use separate mechanisms to determine whether a packet is
in the area (in particular, OSI makes use of area addresses, and IP
determines that a destination is not in an area by looking in the
level 1 forwarding database and determining that no entry exists for
that destination within the area). The route to the nearest level 2
router will result in separate entries in the forwarding database for
OSI and IP. For IP, the route to the nearest attached level 2 router
may be entered in the forwarding database as a default route (i.e., a
route with a subnet mask of all 0).

One approach would be to put the results of each Dijkstra algorithm
in a separate forwarding database. For a router which supports both
level 1 and level 2 routing (including level 2 internal and level 2
external routes), and which supports all four types of service, this
would result in twelve separate forwarding databases for IP.
Implementations may choose to minimize the number of forwarding
databases by combining the information from the multiple Dijkstra
calculations into a single database per supported TOS. This is
discussed in section C.2 below.

The SPF algorithm specified in section C.2.3 of [1] is amended to
appear as follows:

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Step 0: Initialize TENT and PATHS to empty. Initialize tentlength to
[internalmetric=0, externalmetric=0].

(tentlength is the pathlength of elements in TENT that we are
examining.)

1) Add <SELF,0,W> to PATHS, where W is a special value indicating
traffic to SELF is passed up to internal processes (rather than
forwarded).

entry made to TENT must be marked as being either an End System
or a router to enable the check at the end of Step 2 to be made
correctly - Note that each local IP reachability entry is
included as an adjacency, and is marked as being an End System).
IP reachability entries) on enabled circuits, to system N of
SELF in state "Up" compute:

d(N) = cost of the parent circuit of the adjacency (N),
obtained from metric.k , where k = one of {default metric,
delay metric, monetary metric, error metric}

3) If a triple <N,x,{Adj(M)}> is in TENT, then:

4) If N is a router or an OSI End System entry, and there are now
excess adjacencies as described in Clause 7.2.7 of [1]. If N
is an IP Reachability Entry, then excess adjacencies may be
removed as desired. This will not effect the correctness of
routing, but may eliminate the determinism for IP routes (i.e.,
IP packets still follow optimal routes within an area, but
where multiple equally good routes exist, will not necessarily
follow precisely the route that any one particular router
would have anticipated).

5) If x < d(N), do nothing.

6) If x > d(N), remove <N,x,{Adj(M)}> from TENT and add the triple

to TENT.

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8) Now add systems to which the local router does not have adjacencies,
but which are mentioned in neighboring pseudonode LSPs. The
adjacency for such systems is set to that of the designated router.
Note that this does not include IP reachability entries from
neighboring pseudonode LSPs. In particular, the pseudonode LSPs
do not include IP reachability entries.

9) For all broadcast circuits in state "On", find the pseudonode
LSP for that circuit (specifically, the LSP with number zero and
with the first 7 octets of LSPID equal to LnCircuitID for that
circuit, where n is 1 (for level 1 routing) or 2 (level 2
routing)). If it is present, for all the neighbors N reported in
all the LSPs of this pseudonode which do not exist in TENT add
an entry <N,d(N),{Adj(N)}> to TENT, where:

d(N) = metric.k  of the circuit.

10) Go to Step 2.

Step 1: Examine the zeroeth link state PDU of P, the system just
placed on PATHS (i.e., the LSP with the same first 7 octets of LSPID
as P, and LSP number zero).

1) If this LSP is present, and the "Infinite Hippity Cost" bit is
clear, then for each LSP of P (i.e., all LSPs with the same
first 7 octets of LSPID and P, irrespective of the value of
LSP number) compute:

dist(P,N) = d(P) + metric.k(P,N)

for each neighbor N (both End System and router) of the system P. If
the "Infinite Hippity Cost" bit is set, only consider the End System
neighbors of the system P. Note that the End Systems neighbors of the
system P includes IP reachable address entries included in the LSPs
from system P. Here, d(P) is the second element of the triple

and metric.k(P,N) is the cost of the link from P to N as reported in

2) If dist(P,N) > MaxPathMetric, then do nothing.

3) If <N,d(N),{Adj(N)}> is in PATHS, then do nothing.

Note: d(N) must be less than dist(P,N), or else N would not
have been put into PATHS. An additional sanity check may be

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done here to ensure that d(N) is in fact less than dist(P,N)

4) If a triple <N,x,{Adj(N)}> is in TENT, then:

b) If N is a router or an OSI end system, and there are now more
excess adjacencies, as described in clause 7.2.7 of [1]. For
IP Reachability Entries, excess adjacencies may be removed as
desired. This will not effect the correctness of routing, but
may eliminate the determinism for IP routes (i.e., IP packets
will still follow optimal routes within an area, but where
multiple equally good routes exist, will not necessarily follow
precisely the route that any one particular router would have
anticipated).

c) if x < dist(P,N), do nothing.

Step 2: If TENT is empty, stop. Else:

1) Find the element <P,x,{Adj(P)}>, with minimal x as follows:

a) If an element <*,tentlength,*> remains in TENT in the list for
tentlength, choose that element. If there are more than one
elements in the list for tentlength, choose one of the elements
(if any) for a system which is a pseudonode in preference to one
for a non-pseudonode. If there are no more elements in the list
for tentlength, increment tentlength and repeat Step 2.

d) If this is the Level 2 Decision Process running, and the system
just added to PATHS listed itself as Partition Designated Level 2
to PATHS, where AREA.P is the Network Entity Title of the other
end of the Virtual Link, obtained by taking the first AREA
listed in P's LSP and appending P's ID.

e) If the system just added to PATHS was an end system, go to

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step 2. Else go to Step 1.

NOTE - In the level 2 context, the "End Systems" are the set of
zero cost (again, for OSI), plus the set of IP reachability entries
(including both internal and external).

C.2 Forwarding of IP packets

The SPF algorithm specified in section C.1 may be used to calculate
(logically) separate IP forwarding tables for each type of service,
and for level 1, level 2 internal, and level 2 external routes.
Section C.2.1 describes how to forward IP packets, based on these
multiple forwarding databases. Section C.2.2 describes how the
multiple forwarding databases can be combined into a single
forwarding database per supported TOS.

C.2.1 Basic Method for Forwarding IP packets

For level 1-only routers:

- Determine if the IP destination address matches any entry in the
level 1 forwarding table for the specified TOS.

- Determine if the IP destination address matches any entry in the
level 1 forwarding table for the default TOS.

- If default TOS resulted in more specific entry, forward according
to default TOS.

- If equally specific entries found, or specified TOS resulted in
more specific entry, forward according to specified TOS

- If no entry was found (which includes no default route entry), then
destination is unreachable.

Note: For level 1 only routers, the route to the nearest attached
level 2 router will be entered into the forwarding database as a
default route (i.e., a route with a subnet mask which is all 0). Thus
this last event (no entry found) can occur only if there is no
attached level 2 router reachable in the area.

For routers which are both level 1 and level 2 routers:

- Determine if the IP destination address matches any entry in the
level 1 forwarding table for the specified TOS.

- Determine if the IP destination address matches any entry in the

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level 1 forwarding table for the default TOS.

- If default TOS resulted in more specific entry (i.e., more bits in
the subnet mask take the value 1), forward according to default TOS.

- If equally specific entries found, or specified TOS resulted in
more specific entry, forward according to specified TOS

- If no entry found:

- Determine if the IP destination address matches any entry in the
level 2 internal forwarding table for the specified TOS.

- Determine if the IP destination address matches any entry in the
level 2 internal forwarding table for the default TOS.

- If default TOS resulted in more specific entry, forward according
to default TOS.

- If equally specific entries found, or specified TOS resulted in
more specific entry, forward according to specified TOS

- If no entry found:

- Determine if the IP destination address matches any entry in the
level 2 external forwarding table for the specified TOS.

- Determine if the IP destination address matches any entry in the
level 2 external forwarding table for the default TOS.

- If default TOS resulted in more specific entry, forward according
to default TOS.

- If equally specific entries found, or specified TOS resulted in
more specific entry, forward according to specified TOS

- If no entry is found, then destination is unreachable

For level 2-only routers, the above algorithm can be used, except
since there is no level 1 forwarding database, the corresponding
steps can be skipped.

As discussed in section 3.10.2, for level 2 routers which are
announcing manually configured summary addresses in their level 2
LSPs, in some cases there will exist IP addresses which match the
which are reachable via level 1 routing in the area. Packets to such
addresses are handled according to the rules specified in section

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3.10.2. This may be accomplished by adding the manually configured
(for the appropriate TOS), with a special "next hop" address which
specifies that packets for which this entry is selected are to be
discarded. This will work correctly because more desireable entries
(such as delivering the packet via level 1 routing to the correct
destination, or a more specific level 2 route) will automatically
take precedence according to the forwarding rules specified above.
Less desireable routes (such as using a level 2 external route to the
"default route" entry) are not possible because other level 2 routers

C.2.2 Reduction of IP Forwarding Databases

The multiple forwarding databases used in the basic forwarding method
in section C.2.1 can be reduced, by combining the multiple databases
into one database for each supported TOS.

For reduction of IP forwarding databases, it is assumed that for any
two overlapping address entries, either the entries are identical, or
one range contains the other. In other words, for any two [IP
address, subnet mask] entries A and B, if there is at least one IP
address which matches both entries, then either: (i) the two entries
are identical; or (ii) entry A contains entry B (i.e., any IP address
which matches entry B also matches entry A); or (iii) entry B
contains entry A (any IP address which matches entry A also matches
entry B).

Non-contiguous subnet masks can be configured to violate this
assumption. For example, consider the two entries:

In this case neither entry contains the other. Specifically;

- there are IP addresses which match both A and B (e.g.,
"01010101 01010101 xxxxxxxx xxxxxxxx"),

- there are IP addresses which match A but not B (e.g.,
"01010101 11110101 xxxxxxxx xxxxxxxx")

- there are IP addresses which match B but not A (e.g.,
"01010101 01011111 xxxxxxxx xxxxxxxx").

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The reduction of the multiple forwarding databases for each TOS to a
single database for each TOS is based on the use of "best match"
routing, combined with reduction of the entries placed in the
forwarding database in order to eliminate entries which are not to be
selected (based on the order of preference of routes specified in
section 3.10). The specific algorithm for creation of the IP
forwarding database can be described as follows:

1) Make use of the the Dijkstra algorithm described in section C.1 to
create separate forwarding databases for each supported TOS for
level 1 routes, level 2 internal routes, and level 2 external
routes. (Note that each entry in the forwarding database will
next hop router for IP packets which match that entry).

2) For each level 1 route entry which has been placed in the level 1
IP forwarding database for a specific TOS, copy that entry into
the overall IP forwarding database for that TOS.

3) For each route entry X which has been placed in the level 2 internal
IP forwarding database for a specific TOS, search for overlapping
entries in the level 1 IP forwarding database for the specific TOS,
and also for the default TOS:

a) If there is any overlapping entry Y in the level 1 forwarding
database (for the specfic TOS, or for the default TOS) such
that either (i) Y contains X; or (ii) Y is identically specific
to X; then ignore entry X.

b) Otherwise, copy entry X into the overall IP forwarding database
for the specific TOS.

4) For each route entry X which has been placed in the level 2
external IP forwarding database for a specific TOS, search for
overlapping entries in the level 1 IP forwarding database for
the specific TOS, and for the default TOS, and the level 2
internal IP forwarding database for the specific TOS, and for
the default TOS.

a) If there is an overlapping entry Y such that either (i) Y
contains X; or (ii) Y is identically specific to X; then
ignore entry X.

b) Otherwise, copy entry X into the overall IP forwarding database
for the specific TOS.

This method will result in one forwarding database for each supported
TOS. The forwarding of packets can then be simplified to be as follows:

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1) For IP packets which map to the default TOS metric (or to an
unsupported TOS metric), search the default TOS forwarding
database and select the entry which has the most specific match.
Forward the packet accordingly.

2) For packets which map to a specific (non-default) TOS metric,
search the specific TOS forwarding database and select the entry
j which has the most specific match. Also search the default TOS
forwarding database and select the entry k which has the most
specific match. Forward the packet as follows:

a) If k is more specific than j, forward according to entry k

b) If j and k are equally specific, forward according to entry j

c) If j is more specific than k, forward according to entry j

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Annex D
Use of the Authentication Field

The use of the Authentication field is outside of the scope of this
specification. However, there is a urgent need for simple error
detection/authentication mechanisms (such as a simple password) to
protect against certain types of errors. This annex therefore
proposes a possible use of this field.

This annex is included for informational purposes.

D.1 Authentication Field in IS-IS packets

All IS-IS packets may optionally include the authentication field, as
described in sections 3.9 and 5 of this specification. As described
in section 5, the authentication field is encoded as a (Code, Length,
Value) triplet. This annex proposes that the contents of the Value
field consist of a one octet "Authentication Type" field, plus a
variable length "Authentication Information" field. A specific value
of the "Authentication Type" is assigned to passwords, transmitted in
the clear without encryption. The authentication field is encoded as
follows:

7 Authentication Information -- Information used to authenticate
the PDU

x CODE - 133

x LENGTH - total length of the value field

x VALUE -
No. of Octets
+--------------------------------+
|      Authentication Type       |      1
+--------------------------------+
|   Authentication Information   |      VARIABLE
+--------------------------------+

The Authentication Type is assigned as follows:

Type  =  0        reserved

Type  >  1        reserved

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D.2 Authentication Type 1 - Simple Password

Using this authentication type, a variable length password is passed
in the clear (i.e., not encrypted) in the Authentication Information
field.

WARNING: The use of a simple password does not provide useful
protection against intentional misbehavior. In particular, since the
password is transmitted in the clear without encryption, it is easy
for a hostile system to intercept the passwords, and to transmit
authenticated packets. The use of simple passwords should be
considered only as a weak protection against accidental errors such
as accidental misconfiguration.

domain basis. Specifically, when this form of authentication is used:

- IS-IS Hello and 9542 IS Hello packets shall contain the

- Level 1 Sequence Number Packets shall contain the per-area password

- Level 2 Sequence Number Packets shall contain the per-domain

Also, each of these three passwords shall be configured with: (i)
in the receive password set will be accepted on receipt. This method
allows the graceful changing of passwords without temporary loss of
connectivity.

For example, consider the case that an area has the configured area
first step would be to manually configure all of the routers in the
level 1 LSPs and SNPs. However, they will also accept the alternate

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routers in the area to set the per-area transmit password to
will be using the new value of the per-area password, but will accept
the old value as well. Finally, the third step is to change all
routers in the area to have the per-area receive password set to

manner, it is possible to maintain continuous correct operation. For
example, in the middle of the second step in the above example, some
of the routers in the area will be transmitting level 1 LSPs and SNPs
during the second step of the transition all routers in the area will
accept level 1 LSPs and SNPs using either password.

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Annex E
Interaction of the Integrated IS-IS with Brouters

A "brouter" is a device which operates an both a bridge and a router.
One possible type of brouter acts as a router for IP traffic, and
acts as a bridge for all other types of traffic.

Depending upon the manner in which a brouter is implemented, and
depending upon the network topology, there is an obscure bug which
can result from the interaction of the Integrated IS-IS protocol, and
brouters. This appendix gives an example of the bug, and proposes a
simple correction to the operation of brouters to correct the
problem.

This annex is included for informational purposes.

E.1 The Problem

Suppose that we have a brouter which treats IP packets as if it were
a normal IP router, and which treats all other packets as if it is a
bridge.

Suppose that two routers "X" and "Y" (which implement the integrated
IS-IS protocol), two Ethernets, and a brouter B are all connected as
follows:

|                               |
+----+---+                      +----+---+
| Router |                      | Router |
|   X    |                      |   Y    |
+----+---+                      +----+---+
|                               |
-----+------------+-   -+------------+----
|     |
+-+-----+-+
| Brouter |
|    B    |
+---------+

Here suppose that X and Y are running the Integrated IS-IS protocol,
and are both level 1 routers in the same area. Thus X and Y send IS-
IS Hello packets on the LAN. These Hello packets are received and
forwarded by the brouter (using normal bridge functions). Similarly,
X and Y receive each other's IS-IS LSP packets. In this way, it
appears to the Brouter that X and Y are exchanging OSI packets, and
so they are forwarded using normal bridge functions. It appears to X

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and Y as if they are on the same LAN, and so they learn each others
48-bit Ethernet addresses and exchange routing information.

Now, suppose that X receives an IP packet, which it needs to forward
via Y. Since X thinks that it and Y are on the same Ethernet, it just
forwards the IP packet directly, using normal Ethernet encapsulation
and using the 48-bit Ethernet address of Y as the destination address
in the Ethernet header. Brouter B, when thinking as a bridge says:
"this is an IP packet, I don't forward this as a bridge". Brouter B,
when thinking like an IP router says: "this is an IP packet, I know
how to forward IP packets. However, this is sent to an Ethernet
address which is not me, thus I will ignore it". The result is that
the IP packet does not get forwarded.

This problem relates directly to the fact that X and Y are exchanging
OSI packets to determine the connectivity of the path between them,
but then are trying to send IP packets over the path. Also, there is
a device between X and Y on the path which treats OSI and IP packets
differently.

Also note that this problem can also occur in more complex
topologies, whenever a brouter is treating OSI and IP packets in a
fundamentally different manner.

E.2 Possible Solutions

E.2.1 More Sophisticated Brouter

One solution is that brouter B needs to be a little more
sophisticated. In particular, it needs to use the following rules:

- For packets which are not IP packets, act as a bridge (this is the
same as before).

- For IP packets sent to an Ethernet broadcast or multicast address,
act as an IP router (this is also the same as before).

- For IP packets sent to my own Ethernet 48-bit address(es), act as
an IP router (this is also the same as before).

- For IP packets sent to a single station 48-bit address which is not
one of my addresses, act at a bridge (THIS IS NEW).

With this change, the IP packet transmitted from X to Y is forwarded
by the brouter, acting as a bridge. This allows the Brouter and the
multiprotocol routers to interoperate properly.

Callon                                                         [Page 84]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990

E.2.2 Dual Router / Brouter

An alternate solution would be for the Brouter to route both OSI and
IP equally. If the Brouter used the integrated IS-IS for this
purpose, then it could be part of the same routing domain and
interoperate like any other dual router (except for the ability to
bridge other protocol suites).  If it used other protocols for
routing OSI and IP, then it would need to be part of another routing
domain, and could interoperate with integrated IS-IS routers like any
other external router.

Callon                                                         [Page 85]