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Versions: 00 01 02 03 rfc2966                                           
Network Working Group                                                Tony Li
INTERNET DRAFT                                              Juniper Networks
                                                               February 1999

         Domain-wide Prefix Distribution with Multi-Level IS-IS



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

   Internet-Drafts are working documents of the Internet Engineering
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1.0 Abstract

   This document describes extensions to the IS-IS protocol to support
   optimal routing within a multi-level domain.  The IS-IS protocol is
   specified in ISO 10589 [1], with extensions for supporting IPv4
   specified in RFC 1195 [2].

   This document extends the semantics presented in RFC 1195 so that a
   routing domain running with both Level 1 and Level 2 Intermediate
   Systems (IS) [routers] can distribute IP prefixes between Level 1 and
   Level 2 and vice versa.  This distribution requires certain
   restrictions to insure that persistent forwarding loops do not form.
   The goal of this domain-wide prefix distribution is to increase the
   granularity of the routing information within the domain.

2.0 Introduction

   An IS-IS routing domain (a.k.a., an autonomous system running IS-IS)
   can be partitioned into multiple level 1 (L1) areas, and a level 2
   (L2) connected subset of the topology that interconnects all of the
   L1 areas.  Within each L1 area, all routers exchange link state
   information.  L2 routers also exchange L2 link state information to
   compute routes between areas.

   RFC 1195 [2] defines the Type, Length and Value (TLV) tuples that are
   used to transport IPv4 routing information in IS-IS.  RFC 1195 also
   specifies the semantics and procedures for interactions between
   levels.  Specifically, routers in a L1 area will exchange information
   within the L1 area.  For IP destinations not found in the prefixes in
   the L1 database, the L1 router should forward packets to the nearest
   router that is in both L1 and L2 (i.e., an L1L2 router) with the
   'attach' bit set in its L1 Link State Protocol Data Unit (LSP).

   Also per RFC 1195, an L1L2 router should be manually configured with
   a set of prefixes that summarize the IP prefixes found in that L1
   area.  These summaries are injected into L2.  RFC 1195 specifies no
   further interactions between L1 and L2 for IPv4 prefixes.

2.1 Motivations for domain-wide prefix distribution

   The mechanisms specified in RFC 1195 are appropriate in many
   situations, and lead to excellent scalability properties.  However,
   in certain circumstances, the domain administrator may wish to
   sacrifice some amount of scalability and distribute more specific
   information than is described by RFC 1195.  This section discusses
   the various reasons why the domain administrator may wish to make
   such a tradeoff.

   One major reason for distributing more prefix information is to
   improve the quality of the resulting routes.  A well know property of
   prefix summarization or any abstraction mechanism is that it
   necessarily results in a loss of information.  This loss of
   information in turn results in the computation of a route based upon
   less information, which will frequently result in routes that are not

   A simple example can serve to demonstrate this adequately.  Suppose
   that a L1 area has two L1L2 routers that both advertise a single
   summary of all prefixes within the L1 area.  To reach a destination
   inside the L1 area, any other L2 router is going to compute the
   shortest path to one of the two L1L2 routers for that area.  Suppose,
   for example, that both of the L1L2 routers are equidistant from the
   L2 source, and that the L2 source arbitrarily selects one L1L2
   router.  This router may not be the optimal router when viewed from
   the L1 topology.  In fact, it may be the case that the path from the
   selected L1L2 router to the destination router may traverse the L1L2
   router that was not selected.  If more detailed topological
   information or more detailed metric information was available to the
   L2 source router, it could make a more optimal route computation.

   This situation is symmetric in that an L1 router has no information
   about prefixes in L2 or within a different L1 area.  In using the
   nearest L1L2 router, that L1L2 is effectively injecting a default
   route without metric information into the L1 area.  The route
   computation that the L1 router performs is similarly suboptimal.

   Besides the optimality of the routes computed, there is another
   significant driver for the domain wide distribution of prefix
   information.  That driver is the current practice of using the IGP
   (IS-IS) metric as part of the BGP Multi-Exit Discriminator (MED).
   The value in the MED is advertised to other domains and is used to
   inform other domains of the optimal entry point into the current
   domain.  Current practice is to take the IS-IS metric and insert it
   as the MED value.  This tends to cause external traffic to enter the
   domain at the point closest to the exit router.  Note that the
   receiving domain may, based upon policy, choose to ignore the MED
   that is advertised.  However, current practice is to distribute the
   IGP metric in this way in order to optimize routing wherever
   possible.  This is possible in current networks that only are a
   single area, but becomes problematic if hierarchy is to be installed
   into the network.  This is again because the loss of end-to-end
   metric information means that the MED value will not reflect the true
   distance across the advertising domain.  Full distribution of prefix
   information within the domain would alleviate this problem as it
   would allow accurate computation of the IS-IS metric across the
   domain, resulting in an accurate value presented in the MED.

2.2 Scalability

   The disadvantage to performing the domain-wide prefix distribution
   described above is that it has an impact to the scalability of IS-IS.
   Areas within IS-IS help scalability in that LSPs are contained within
   a single area.  This limits the size of the link state database, that
   in turn limits the complexity of the shortest path computation.

   Further, the summarization of the prefix information aids scalability
   in that the abstraction of the prefix information removes the sheer
   number of data items to be transported and the number of routes to be

   It should be noted quite strongly that the distribution of prefixes
   on a domain wide basis impacts the scalability of IS-IS in the second
   respect.  It will increase the number of prefixes throughout the
   domain.  This will result in increased memory consumption,
   transmission requirements and computation requirements throughout the

   It must also be noted that the domain-wide distribution of prefixes
   has no effect whatsoever on the first aspect of scalability, namely
   the existence of areas and the limitation of the distribution of the
   link state database.

   Thus, the net result is that the introduction of domain-wide prefix
   distribution into a formerly flat, single area network is a clear
   benefit to the scalability of that network.  However, it is a
   compromise and does not provide the maximum scalability available
   with IS-IS.  Domains that choose to make use of this facility should
   be aware of the tradeoff that they are making between scalability and
   optimality and provision and monitor their networks accordingly.
   Normal provisioning guidelines that would apply to a fully
   hierarchical deployment of IS-IS will not apply to this type of

4.0 New semantics for external type metrics

   RFC 1195 defines two TLVs for carrying IP prefixes.  TLV 128 is
   defined to carry 'internal' prefixes and TLV 130 is defined to carry
   'external' prefixes.  The original intent of RFC 1195 was to carry
   intra-domain routes within the internal prefix TLV and inter-domain
   routes or intra-domain routes from alternate IGPs in an external
   prefix TLV.  Interestingly, TLV type 130 is not documented to exist
   in Level 2 LSPs.

   In addition to this distinction, RFC 1195 provides for a bit in each
   of these TLVs that distinguishes between an internal metric type and
   an external metric type.  Similarly, the clear intent was that the
   internal metric type should reflect a total metric that is the sum of
   the metrics to the advertising router plus the metric to the prefix.
   Further, for an external metric type, the total metric should simply
   be the metric advertised to the prefix, not including the total
   metric necessary to reach the exit router.  Prefixes with internal
   metrics are always preferred over external metrics, regardless of the
   value of the metrics.

   It should be noted that the combination of an internal prefix with an
   external metric type is not obviously useful, and is not well defined
   by RFC 1195.

   It should also be noted that as of this writing, the author knows of
   no deployed implementations that make use of either the external
   prefix or the external metric type.  The implication is that this
   proposal is free to redefine the semantics of the external metric
   type without conflict.

   An essential property when redistributing prefixes between levels is
   to insure that no persistent loops form in the distribution of
   information (i.e., a routing loop), as this would lead to the
   indefinite propagation of the information, even in the event that the
   information was no longer originated by some system in the domain.
   Further, a routing loop is likely to form a forwarding loop, where
   actual traffic traverses the network in a cycle in the topology.
   Forwarding loops are known to consume large amounts of resources and
   are to be avoided.

4.1 Proposed semantics for the external metric type

   To provide the above properties, this proposal defines the following

   1) Only internal metric type prefixes are redistributed from L1 into
   L2, and these will be marked as an external metric type when
   advertised into L2.

   2) All prefixes can be redistributed from L2 into L1 but again will
   be marked as an external metric type when advertised into L1.

   3) Within L1, a route to a prefix with an internal metric type is
   preferred over a route to the same prefix with an external metric
   type, regardless of the comparison of the metrics.

   Based on these rules, we first observe that this proposal is free
   from routing loops.  No prefix can be redistributed from L2 to L1 and
   back into L2, because the route is marked with external metric type
   in L1 and by rule 1 cannot be redistributed into L2.  Similarly, a
   prefix redistributed from L1 to L2 and back into the original L1 area
   will not be used while an L1 internal metric type prefix is
   available.  There is the possibility of a transient routing loop in
   this situation when the original prefix is withdrawn and the external
   prefix is selected.  However, all link state protocols are subject to
   transient routing loops, so this is no worse than the status quo.

   Note that this proposal is not radically different than the current
   semantics for RFC 1195: internal metric types are always preferred
   over externals, so rule (3) is an extension that allows external
   metric types in internal prefix TLVs.  It does not introduce a new
   comparison between internal and external metric types.

4.2 Transition issues

   Because no implementations currently make use of the external metric
   type, the deployment of prefixes with an external metric type is
   somewhat problematic.  There is the possibility that the new type of
   advertisement may result in software instability in systems that do
   not deal with even the original semantics correctly.  Further, there
   is a danger that haphazard deployment of systems supporting this
   proposal and legacy systems would have an unfortunate interaction.
   It is recommended, for any L1 area that should perform the mutual
   redistribution described in this proposal, that the L1L2 systems be
   updated first.  If these systems operate correctly, this is
   sufficient to insure that there are no persistent routing loops.

5.0 Comparisons with other proposals

   There are two other proposals currently being discussed which are
   similar to this proposal in nature.  This section discusses each of
   these proposals and their relationship to this proposal.

5.1 Creation of new TLVs

   In [3], a new TLV is proposed to transport IP prefix information.
   Because this is a new TLV, it is somewhat harder to deploy, requiring
   that all systems understand the new TLV before it can become
   effective.  For this reason, this proposal provides an alternative
   that can be deployed sooner.  There is no effective semantic
   difference between the two proposals.  In [3], a bit is defined to
   mark a prefix as 'up' or 'down'.  This is essentially the same
   semantics as is proposed here.

5.2 Usage of external prefixes

   An alternate proposal, [4], also uses effectively the same semantics,
   but encodes the information somewhat differently.  Prefixes that
   would be marked with the external metric type would be instead
   encoded as external prefixes.  This forces the usage of a separate
   TLV, resulting in a few extra bytes of overhead.  This is not a
   significant difference.  The primary differences are syntactic, and
   the addition of the external prefix TLV to the L1 LSP.  The latter is
   a clear omission in RFC 1195 and should have been in the original

6.0 Security Considerations

   This document raises no new security issues for IS-IS.

7.0 Acknowledgments

   The authors would like to thank Henk Smit for his comments on this

8.0 References

   [1] ISO 10589, "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)" [Also republished as RFC 1142]

   [2] RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual
   environments", R.W. Callon, Dec. 1990

   [3] Smit, H., Li, T. "IS-IS extensions for Traffic Engineering",
   draft-ietf-isis-traffic-00.txt, work in progress

   [4] Patel, A., Przygienda, T., "L1/L2 Optimal IS-IS Routing", draft-
   ietf-isis-l1l2-00.txt, work in progress

9.0 Author's Address

   Tony Li
   Juniper Networks, Inc.
   385 Ravendale Dr.
   Mountain View, CA 94043
   Email: tli@juniper.net
   Fax: +1 650 526 8001
   Voice: +1 650 526 8006