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Versions: 00 01 02 RFC 3787

Internet Engineering Task Force                        J. Parker, Editor
                                                       Axiowave Networks

                                                        December 9, 2003

       Recommendations for Interoperable IP Networks using IS-IS

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   Copyright Notice Copyright (C) The Internet Society (2003).  All
   Rights Reserved.

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          This document discusses a number of differences between the
          IS-IS protocol used to route IP traffic as described in RFC
          1195 and the protocol as it is deployed today.  These
          differences are discussed as a service to those implementing,
          testing, and deploying the IS-IS Protocol to route IP traffic.
          A companion document describes the differences between the
          protocol described in ISO 10589 and current practice.

Table of Contents

    1.  Introduction.........................................    2
    2.  Acknowledgments......................................    3
    3.  Unused Features......................................    3
    4.  Overload Bit.........................................    4
    5.  Migration from Narrow Metrics to Wide................    5
    6.  Intermediate System Hello (ISH) PDU..................    7
    7.  Attached Bit.........................................    8
    8.  Default Route........................................    8
    9.  Non-homogeneous Protocol Networks....................    9
   10.  Adjacency Creation and IP Interface Addressing.......    9
   11.  Security Considerations.............................    10
   12.  Normative References.................................   10
   13.  Informative References...............................   11
   14.  Author's  Address....................................   11
   15.  Full Copyright Statement.............................   11

1. Overview

   Interior Gateway Protocols such as IS-IS are designed to provide
   timely information about the best routes in a routing domain.  The
   original design of IS-IS, as described in ISO 10589 [1] has proved to
   be quite durable.  However, a number of original design choices have
   been modified. This document describes some of the differences
   between the protocol as described in RFC 1195 [2] and the protocol
   that can be observed on the wire today. A companion document
   describes the differences between the protocol described in ISO 10589
   and current practice.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
   this document are to be interpreted as described in RFC 2119 [3].

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2. Acknowledgments

   This document is the work of many people, and is the distillation of
   over a thousand mail messages.  Thanks to Vishwas Manral, who pushed
   to create such a document.  Thanks to Danny McPherson, the original
   editor, for kicking things off.  Thanks to Mike Shand, for his work
   in creating the protocol, and his uncanny ability to remember what
   everything is for.  Thanks to Micah Bartell and Philip Christian, who
   showed us how to document difference without displaying discord.
   Thanks to Les Ginsberg, Neal Castagnoli, Jeff Learman, and Dave Katz,
   who spent many hours educating the editor. Thanks to Radia Perlman,
   who is always ready to explain anything. Thanks to Satish Dattatri,
   who was tenacious in seeing things written up correctly, and to Bryan
   Boulton for his work on the IP adjacency issue.  Thanks to Russ
   White, whose writing improved the treatment of every topic he
   touched.  Thanks to Shankar Vemulapalli, who read several drafts with
   close attention. Thanks to Don Goodspeed, for his close reading of
   the text. Thanks to Michael Coyle for identifying the quotation from
   Jan L.A. van de Snepscheut.  Thanks for Alex Zinin's ministrations
   behind the scenes.  Thanks to Tony Li and Tony Przygienda, who kept
   us on track as the discussions veered into the weeds. And thanks to
   all those who have contributed, but whose names I have carelessly
   left from this list.

3. Unused Features

   Some features defined in RFC 1195 are not in current use.

3.1 Inter-Domain Routing Protocol Information TLV, Code 131

   RFC 1195 defines an Inter-Domain Routing Protocol Information TLV,
   with code 131, designed to convey information transparently between
   boundary routers. TLV 131 is not used, and MUST be ignored if

3.2 Authentication TLV, Code 133

   RFC 1195 defines an authentication TLV, code 133, which contains
   information used to authenticate the PDU. This TLV has been replaced
   by TLV 10, described in "IS-IS Cryptographic Authentication" [4].
   TLV 133 is not used, and MUST be ignored.

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4. Overload Bit

   To deal with transient problems that prevent an IS from storing all
   the LSPs it receives, ISO 10589 defines an LSP Database Overload
   condition in section 7.3.19.  When an IS is in Database Overload
   condition, it sets a flag called the Overload Bit in the non-
   pseudonode LSP number Zero that it generates. Section of ISO
   10589 instructs other systems not to use the overloaded IS as a
   transit router. Since the overloaded IS does not have complete
   information, it may not be able to compute the right routes, and
   routing loops could develop.  However, an overloaded router may be
   used to reach End Systems directly attached to the router, as it may
   provide the only path to an End System.

   The ability to signal reduced knowledge is so useful that the meaning
   of this flag has been overloaded. In a Service Provider's network,
   when a router running BGP and IS-IS reboots, BGP might take more time
   to converge than IS-IS. Thus the router may drop traffic for
   destinations not yet learned via BGP. It is convenient to set the
   Overload Bit until BGP has converged, as described in "Intermediate
   System to Intermediate System (IS-IS) Transient Blackhole Avoidance"

   An implementation SHOULD use the Overload Bit to signal that it is
   not ready to accept transit traffic.

   An implementation SHOULD not set the Overload bit in PseudoNode LSPs
   that it generates, and Overload bits seen in PseudoNode LSPs SHOULD
   be ignored. This is also discussed in the companion document on ISO

   RFC 1195 makes clear when describing the SPF algorithm for IP routers
   in section C.1.4 that directly connected IP subnetworks are reachable
   when an IS is overloaded.

          Note that the End Systems neighbors of the system P includes
          IP reachable address entries included in the LSPs from system

   When processing LSPs received from a router which has the Overload
   bit set in LSP number Zero, the receiving router SHOULD treat all IP
   reachability advertisements as directly connected and use them in its
   SPF computation.

   Since the IP prefixes that an overloaded router announces will be
   treated as directly attached, an overloaded router SHOULD take care
   in selecting which routes to advertise in the LSPs it generates.

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5. Migration from Narrow Metrics to Wide

   The IS-Neighbors TLV (TLV 2) as defined in ISO 10589 and the IP
   Reachability TLV (TLV 128/TLV 130) as defined in RFC 1195 provide a 6
   bit metric for the default link metric to the listed neighbor.  This
   metric has proved too limited.  The Extended IS-Neighbors TLV (TLV
   22) and the Extended IP Reachability TLV (TLV 135) are defined in
   "IS-IS extensions for Traffic Engineering" [5].  The Extended IS-
   Neighbors TLV (TLV 22) defines a 24 bit metric, and the Extended IP
   Reachability TLV (TLV 135) defines a 32 bit metric for IP Networks
   and Hosts.

   If not all devices in the IS-IS domain support wide metrics, narrow
   metrics MUST continue to be used. Once all devices in the network are
   able to support the new TLVs containing wide metrics, the network can
   be migrated to the new metric style, though care must be taken to
   avoid routing loops.

   We make the following assumptions about the implementation:

          (1)  Each system can generate and understand both narrow and
               wide metrics.

          (2)  The implementation can run the SPF algorithm on an LSP DB
               with instances of both metric styles.

          (3)  If there are two metric styles for a link or IP prefix,
               it will pick one of them as the true cost for the link.

   To compare the different variants of the narrow metric with wide
   metrics, we need an algorithm that translates External and Internal
   narrow metrics into a common integer range. Since we have different
   computations for the L1 and L2 routes, we only need to map metrics
   from a single level.

   In RFC 1195 section 3.10.2, item 2c) states that The IP prefixes
   located in "IP External Reachability" with internal-metric and IP
   prefixes located in "IP Internal Reachability" with internal-metric
   have the same preference.  As defined in "Domain-wide Prefix Distri-
   bution with Two-Level IS-IS", the Most Significant Bit on an L1
   metric tells us if the route has been leaked down, but does not
   change the distance. Thus we will ignore the MSBit.

   We interpret the default metric as an 7 bit quantity.  Metrics with
   the external bit set are interpreted as metrics in the range
   [64..127]. Metrics with the external bit clear are interpreted as
   metrics in the range [0..63].

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5.1 Transition Algorithm

   To facilitate a smooth transition between the use of narrow metrics
   exclusively to the use of wide metrics exclusively, the following
   steps must be taken, in the order below.

     (1)  All routers advertise Narrow Metrics as defined in ISO 10589,
          and consider narrow metrics only in their SPF computation.

     (2)  Each system is configured in turn to send wide metrics as well
          as narrow metrics.  The two metrics for the same link or IP
          prefix SHOULD agree.

     (3)  When all systems are advertising wide metrics, make any
          changes necessary on each system to consider Wide Metrics dur-
          ing the SPF, and change MaxPathMetric to 0xFE000000.

     (4)  Each system is configured in turn to stop advertising narrow

     (5)  When the network is only using wide metrics, metrics on indi-
          vidual links may be rescaled to take advantage of the larger

5.2 Dealing with Non-Equal Metrics

   The algorithm above assumes that the metrics are equal, and thus
   needs to make no assumption about which metric the SPF algorithm
   uses.  This section describes the changes that should be made to the
   SPF algorithm when both Narrow and Wide metric styles should be con-
   sidered. Using a common algorithm allows different implementations to
   compute the same distances independently, even if the wide and narrow
   metrics do not agree.

   The standard SPF algorithm proceeds by comparing sums of link costs
   to obtain a minimal cost path. During transition, there will be more
   than one description of the same links.  We resolve this by selecting
   the minimum metric for each link. This may give us a path with some
   links chosen due to a wide metric and some links chosen due to a nar-
   row metric.

   The description below is more complex than the implementation needs
   to be: the implementation may simply select the minimal cost neighbor
   in TENT, discarding paths to destinations we have already reached, as
   described in ISO 10589.

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   The variables MaxPathMetric and MaxLinkMetric SHOULD retain the
   values defined in Table 2 of section 8 of ISO 10589.

   In C.2.5 Step 0 of the description of the SPF algorithm, section b)

          d(N) = cost of the parent circuit of the adjacency N

          If multiple styles of metric for the link are defined, the
          cost will be the minimum available cost for the circuit.

   In C.2.5 Step 0 of the description of the SPF algorithm, section i)

          d(N) = metric of the circuit

          If multiple styles of metric for the link are defined, the
          cost will be the minimum available cost for the circuit.

   In C.2.6 Step 1 of the description of the SPF algorithm, section a)

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

          If multiple styles of metric for the neighbor are defined, the
          cost will be the minimum available cost for the circuit.

6. Intermediate System Hello (ISH) PDU

   The original intent of RFC 1195 was to provide a routing protocol
   capable of handling both CLNS and IPv4 reachability information.  To
   allow CLNS Endstations (ES) to know that they are attached to a
   router, Intermediate Systems are required to send Intermediate System
   Hello PDUs (ISH) for End Stations when a point-to-point circuit comes
   up.  Furthermore, an IS is not allowed to send Intermediate System to
   Intermediate System Hello PDUs (IIH) before receiving an ISH from a
   peer.  This reduces routing protocol traffic on links with a single

   For this reason section 5.1 RFC 1195 states:

          "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 ISHs on their
          point-to-point links."

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   Section 5.1 RFC 1195 reinforces the need to comply with section 8.2.4
   of ISO 10589.  However, in an IP Only environment, the original need
   for the ISH PDU is not present.

   A multi-protocol IS that supports the attachment of CLNS ESs over
   Point to Point circuits must act in accordance with section 8.2.2 ISO
   10589 when CLNS functionality is enabled.

   An IP only implementation SHOULD issue an ISH PDU as described in
   section 8.2.3 of ISO 10589.  This is to inter-operate with implemen-
   tations which require an ISH to initiate the formation of an IS-IS

   An IP Only implementation may issue an IIH PDU when a point to point
   circuit transitions into an "Up" state to initiate the formation of
   an IS-IS adjacency, without sending an ISH PDU.  However, this may
   not inter-operate with implementations which require an ISH for adja-
   cency formation.

   An IS may issue an IIH PDU in response to the receipt of an IIH PDU
   in accordance with section ISO 10589, even though it has not
   received an ISH PDU.

7. The Attached Bit

   In section of ISO 10589, an algorithm is described to deter-
   mining when the attachedFlag should be set on an intermediate system.
   Some implementations also allow the attachedFlag to be set on Inter-
   mediate Systems routing IP traffic when there is a default route in
   the local routing table, or when some other state is reached that
   implies a connection to the rest of the network.

8. Default Route

   RFC 1195 states in section 1.3:

          Default routes are permitted only at level 2 as external
          routes (i.e., included in the "IP External Reachability Infor-
          mation" field, as explained in sections 3 and 5).  Default
          routes are not permitted at level 1.

   Because of the utility of the default route when dealing with other
   routing protocols and the ability to influence the exit point from an
   area, an implementation MAY generate default routes in Level 1.

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9. Non-homogeneous Protocol Networks

   RFC 1195 assumes that every deployment of IS-IS routers will sup-
   port a homogeneous set of protocols. It anticipates OSI only, IP
   only, or dual OSI and IP routers. While it allows mixed areas with,
   for example, both pure IP and Dual IP and OSI routers, it allows only
   IP traffic in such domains, and OSI traffic only when pure OSI and
   Dual IP and OSI routers are present. Thus it provides only lowest
   common denominator routing.

   RFC 1195 also requires the inclusion of the Protocol Supported TLV
   with code 129 in IIH and ISH PDUs and in LSP number Zero. IP capable
   routers MUST generate a Protocol Supported TLV, and MUST include the
   IP protocol as a supported protocol. A router that does not include
   the Protocols Supported TLV may be assumed to be a pure OSI router
   and can be interpreted as implicitly "advertising" support for the
   OSI protocol.

   The requirements of RFC 1195 are ample if networks adhere to this
   restriction. However, the behavior of mixed networks that do not fol-
   low these guidelines is not well defined.

   The ITU-T requires that SONET/SDH equipment running the IS-IS proto-
   col must not form an adjacency with a neighbour unless they share at
   least one network layer protocol in common. Unless this feature is
   present in every IS in the SONET or SDH DCN network the network may
   not function correctly. Implementors MAY include this feature if they
   wish to ensure interoperability with SONET and SDH DCN networks.

   Definition of an interoperable strategy for resolving the problems
   that arise in non-homogeneous protocol networks remains incomplete.
   Members of the ITU are actively working on a proposal: see "Architec-
   ture and Specification of Data Communication Network", [7].

10. Adjacency Creation and IP Interface Addressing

   RFC 1195 states that adjacencies are formed without regard to IP
   interface addressing. However, many current implementations refuse
   adjacencies based on interface addresses and related issues.

   In section 4.2, RFC 1195 requires routers with IP interface addresses
   to advertise the addresses in an IP Interface Address TLV (132) car-
   ried in IIH PDUs. Some implementations will not interoperate with a
   neighbor router that does not include the IP Interface Address TLV.
   Further, some implementations will not form an adjacency on broadcast
   interfaces with a peer who does not share an interface address in
   some common IP subnetwork.

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   If a LAN contains a mixture of implementations, some that form adja-
   cencies with all neighbors and some that do not, care must be taken
   when assigning IP addresses.  If not all routers in a LAN are on the
   same IP subnet, it is possible that DIS election may fail, leading to
   the election of multiple DISs on a LAN, or no DIS at all. Even if DIS
   election succeeds, black holes can result because the IS-IS LAN tran-
   sitivity requirements of section 6.7.3 ISO 10589 are not met.

   Unnumbered point to point links do not have IP interface addresses,
   though they may have other IP addresses assigned to the routers.  The
   IP address assigned to two routers that are neighbors on an unnum-
   bered point to point link do not need to be related.  However, some
   implementations will not form an adjacency on numbered point to point
   links if the interface addresses of each endpoint are not in the same
   IP subnetwork.  This means that care must be taken in assigning IP
   interface addresses in all networks.

   For an implementation to interoperate in a such mixed environment, it
   MUST include an IP Interface address (TLV 132) in its IIH PDUs. The
   network administrator should ensure that there is a common IP subnet
   assigned to links with numbered interfaces, and that all routers on
   each link have a IP Interface Addresses belonging to the assigned

11. Security Considerations

   The clarifications in this document do not raise any new security
   concerns, as there is no change in the underlying protocol described
   in ISO 10589 [1] and RFC 1195 [2].

   The document does make clear that TLV 133 has been deprecated and
   replaced with TLV 10.

12. Normative References

[1]  ISO, "Intermediate system to Intermediate system routeing informa-
     tion exchange protocol for use in conjunction with the Protocol for
     providing the Connectionless-mode Network Service (ISO 8473),"
     ISO/IEC 10589:2002.

[2]  Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
     December 1990.

[3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

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[4]  Li, T., Atkinson, R. J., "IS-IS Cryptographic Authentication", RFC
     3567 July 2003.

[5]  Li, T., Smit, H., "IS-IS extensions for Traffic Engineering",
     draft-ietf-isis-traffic-05.txt, August 2003.

[6]  August 2001.  McPherson, D., "Intermediate System to Intermediate
     System (IS-IS) Transient Blackhole Avoidance", RFC 3277, April

13. Informative References

[7]  ITU, "Architecture and Specification of Data Communication Net-
     work", ITU-T Recommendation G.7712/Y.1703, November 2001

14. Author's Address

   Jeff Parker
   Axiowave Networks
   200 Nickerson Road
   Marlborough, Mass 01752
   e-mail: jparker@axiowave.com

15. Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of develop-
   ing Internet standards in which case the procedures for copyrights
   defined in the Internet Standards process must be followed, or as
   required to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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