Network Working Group                                              T. Li
Request for Comments: 2966                              Procket Networks
Category: Informational                                    T. Przygienda
                                                                 Redback
                                                                 H. Smit
                                                        Procket Networks
                                                            October 2000


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

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes extensions to the Intermediate System to
   Intermediate System (IS-IS) protocol to support optimal routing
   within a two-level domain.  The IS-IS protocol is specified in ISO
   10589, with extensions for supporting IPv4 (Internet Protocol)
   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.

1. 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
   "attached 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 summarizes the IP prefixes reachable in that
   L1 area.  These summaries are injected into L2.  RFC 1195 specifies
   no further interactions between L1 and L2 for IPv4 prefixes.

1.1 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
   optimal.

   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 are two other
   significant drivers for the domain wide distribution of prefix
   information.

   When a router learns multiple possible paths to external destinations
   via BGP, it will select only one of those routes to be installed in
   the forwarding table.  One of the factors in the BGP route selection
   is the IGP cost to the BGP next hop address.  Many ISP networks
   depend on this technique, which is known as "shortest exit routing".
   If a L1 router does not know the exact IGP metric to all BGP speakers
   in other L1 areas, it cannot do effective shortest exit routing.

   The third 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.

1.2 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
   computed.

   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
   domain.

   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
   configuration.

2. Proposed syntax and semantics for L2->L1 inter-area routes

   This document defines the syntax of how to advertise level 2 routes
   in level 1 LSPs.  The encoding is an extension of the encoding in RFC
   1195.

   To some extent, in IS-IS the level 2 backbone can be seen as a
   separate area itself.  RFC 1195 defines that L1L2 routers can
   advertise IP routes that were learned via L1 routing into L2.  These
   routes can be regarded as inter-area routes.  RFC 1195 defines that
   these L1->L2 inter-area routes must be advertised in L2 LSPs in the
   "IP Internal Reachability Information" TLV (TLV 128).  Intra-area L2
   routes are also advertised in L2 LSPs in an "IP Internal Reachability
   Information" TLV.  Therefore, L1->L2 inter-area routes are
   indistinguishable from L2 intra-area routes.

   RFC 1195 does not define L2->L1 inter-area routes.  A simple
   extension would be to allow a L1L2 router to advertise routes learned
   via L2 routing in its L1 LSP.  However, to prevent routing-loops,
   L1L2 routers must never advertise L2->L1 inter-area routes that they

   learn via L1 routing, back into L2.  Therefore, there must be a way
   to distinguish L2->L1 inter-area routes from L1 intra-area routes.
   Draft-ietf-isis-traffic-01.txt defines the "up/down bit" for this
   purpose.  RFC 1195 defines TLVs 128 and 130 to contain IP routes.
   TVLs 128 and 130 have a metric field that consists of 4 TOS metrics.
   The first metric, the so-called "default metric", has the high-order
   bit reserved (bit 8).  Routers must set this bit to zero on
   transmission, and ignore it on receipt.

   This document redefines this high-order bit in the default metric
   field in TLVs 128 and 130 to be the up/down bit.  L1L2 routers must
   set this bit to one for prefixes that are derived from L2 routing and
   are advertised into L1 LSPs.  The bit must be set to zero for all
   other IP prefixes in L1 or L2 LSPs.  Prefixes with the up/down bit
   set that are learned via L1 routing, must never be advertised by L1L2
   routers back into L2.

2.1 Clarification of external route-type and external metric-type

   RFC 1195 defines two TLVs for carrying IP prefixes.  TLV 128 is
   defined as "IP Internal Reachability Information", and should be used
   to carry IP prefixes that are directly connected to IS-IS routers.
   TLV 130 is defined as "IP External Reachability Information", and
   should be used to carry routes learned from outside the IS-IS domain.
   RFC 1195 documents TLV type 130 only for level 2 LSPs.

   RFC 1195 also defines two types of metrics.  Metrics of the internal
   metric-type should be used when the metric is comparable to metrics
   used to weigh links inside the ISIS domain.  Metrics of the external
   metric-type should be used if the metric of an IP prefix cannot be
   directly compared to internal metrics. External metric-type can only
   be used for external IP prefixes.  A direct result is that metrics of
   external metric-type should never be seen in TLV 128.

   To prevent confusion, this document states again that when a router
   computes IP routes, it must give the same preference to IP routes
   advertised in an "IP Internal Reachability Information" TLV and IP
   routes advertised in an "IP External Reachability Information" TLV.
   RFC 1195 states this quite clearly in the note in paragraph 3.10.2,
   item 2c).  This document does not alter this rule of preference.

       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.

   However, IP routes advertised in "IP External Reachability
   Information" with external metric-type must be given less preference
   than the same IP routes advertised with internal-metric type,
   regardless of the value of the metrics.

   While IS-IS routers must not give different preference to IP prefixes
   learned via "IP Internal Reachability Information" and "IP External
   Reachability Information" when executing the Dijkstra calculation,
   routers that implement multiple IGPs are free to use this distinction
   between internal and external routes when comparing routes derived
   from different IGPs for inclusion in their global RIB.

2.2 Definition of external IP prefixes in level 1 LSPs

   RFC 1195 does not define the "IP External Reachability Information"
   TLV for L1 LSPs.  However, there is no reason why an IS-IS
   implementation could not allow for redistribution of external routes
   into L1.  Some IS-IS implementations already allow network
   administrators to do this.  This document loosens the restrictions in
   RFC 1195, and allows for the inclusion of the "IP External
   Reachability Information" TLV in L1 LSPs.

   RFC 1195 defines that IP routes learned via L1 routing must always be
   advertised in L2 LSPs in a "IP Internal Reachability Information"
   TLV.  Now that this document allows "IP External Reachability
   Information" TLVs in L1 LSPs, and allows for the advertisement of
   routes learned via L2 routing into L1, the above rule needs a
   extensions.

   When a L1L2 router advertises a L1 route into L2, where that L1 route
   was learned via a prefix advertised in a "IP External Reachability
   Information" TLV, that L1L2 router should advertise that prefix in
   its L2 LSP within an "IP External Reachability Information" TLV.  L1
   routes learned via an "IP Internal Reachability Information" TLV
   should still be advertised within a "IP Internal Reachability
   Information" TLV.  These rules should also be applied when
   advertising IP routes derived from L2 routing into L1. Of course in
   this case also the up/down bit must be set.

   RFC 1195 defines that if a router sees the same external prefix
   advertised by two or more routers with the same external metric, it
   must select the route that is advertised by the router that is
   closest to itself.  It should be noted that now that external routes
   can be advertised from L1 into L2, and vice versa, that the router
   that advertises an external prefix in its LSP might not be the router
   that originally injected this prefix into the IS-IS domain.
   Therefore, it is less useful to advertise external routes with
   external metrics into other levels.

3. Types of IP routes in IS-IS and their order of preference

   RFC 1195 and this document defines several ways of advertising IP
   routes in IS-IS. There are four variables involved.

   1) The level of the LSP in which the route is advertised.  There are
      currently two possible values: level 1 and level 2
   2) The route-type, which can be derived from the type of TLV in which
      the prefix is advertised.  Internal routes are advertised in IP
      Internal Reachability Information TLVs (TLV 128), and external
      routes are advertised in IP External Reachability Information TLVs
      (TLV 130).
   3) The metric-type: Internal or External. The metric-type is derived
      from the Internal/External metric-type bit in the metric field
      (bit 7).
   4) The fact whether this route is leaked down in the hierarchy, and
      thus can not be advertised back up.  This information can be
      derived from the newly defined up/down bit in the default metric
      field.

3.1 Overview of all types of IP prefixes in IS-IS Link State PDUs

   The combination IP Internal Reachability Information and external
   metric-type is not allowed. Also the up/down bit is never set in L2
   LSPs.  This leaves us with 8 different types of IP advertisements in
   IS-IS.  However, there are more than 8 reasons for IP prefixes to be
   advertised in IS-IS.  The following tables describe the types of IP
   prefixes and how they are encoded.

   1) L1 intra-area routes

    These are advertised in L1 LSPs, in TLV 128.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are directly connected to the advertising router.

   2) L1 external routes

    These are advertised in L1 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are learned from other IGPs, and are usually not
    directly connected to the advertising router.

   3) L2 intra-area routes

    These are advertised in L2 LSPs, in TLV 128.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are directly connected to the advertising router.
    These prefixes can not be distinguished from L1->L2 inter-area
    routes.

   4) L2 external routes

    These are advertised in L2 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are learned from other IGPs, and are usually not
    directly connected to the advertising router.  These prefixes can
    not be distinguished from L1->L2 inter-area external routes.

   5) L1->L2 inter-area routes

    These are advertised in L2 LSPs, in TLV 128.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are learned via L1 routing, and were derived
    during the L1 SPF computation from prefixes advertised in L1 LSPs in
    TLV 128.  These prefixes can not be distinguished from L2 intra-area
    routes.

   6) L1->L2 inter-area external routes

    These are advertised in L2 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is internal metric.
    These IP prefixes are learned via L1 routing, and were derived
    during the L1 SPF computation from prefixes advertised in L1 LSPs in
    TLV 130.  These prefixes can not be distinguished from L2 external
    routes.

   7) L2->L1 inter-area routes

    These are advertised in L1 LSPs, in TLV 128.
    The up/down bit is set to one, metric-type is internal metric.
    These IP prefixes are learned via L2 routing, and were derived
    during the L2 SPF computation from prefixes advertised in TLV 128.

   8) L2->L1 inter-area external routes

    These are advertised in L1 LSPs, in TLV 130.
    The up/down bit is set to one, metric-type is internal metric.
    These IP prefixes are learned via L2 routing, and were derived
    during the L2 SPF computation from prefixes advertised in L2 LSPs in
    TLV 130.

   9) L1 external routes with external metric

    These are advertised in L1 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is external metric.
    These IP prefixes are learned from other IGPs, and are usually not
    directly connected to the advertising router.

   10) L2 external routes with external metric

    These are advertised in L2 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is external metric.
    These IP prefixes are learned from other IGPs, and are usually not
    directly connected to the advertising router.  These prefixes can
    not be distinguished from L1->L2 inter-area external routes with
    external metric.

   11) L1->L2 inter-area external routes with external metric

    These are advertised in L2 LSPs, in TLV 130.
    The up/down bit is set to zero, metric-type is external metric.
    These IP prefixes are learned via L1 routing, and were derived
    during the L1 SPF computation from prefixes advertised in L1 LSPs in
    TLV 130 with external metrics.  These prefixes can not be
    distinguished from L2 external routes with external metric.

   12) L2->L1 inter-area external routes with external metric

    These are advertised in L1 LSPs, in TLV 130.
    The up/down bit is set to one, metric-type is external metric.
    These IP prefixes are learned via L2 routing, and were derived
    during the L1 SPF computation from prefixes advertised in L2 LSPs in
    TLV 130 with external metrics.

3.2 Order of preference for all types of IP routes in IS-IS

   Unfortunately IS-IS cannot depend on metrics alone for route
   selection.  Some types of routes must always preferred over others,
   regardless of the costs that were computed in the Dijkstra
   calculation.  One of the reasons for this is that inter-area routes
   can only be advertised with a maximum metric of 63.  Another reason
   is that this maximum value of 63 does not mean infinity (e.g. like a
   hop count of 16 in RIP denotes unreachable).  Introducing a value for
   infinity cost in IS-IS inter-area routes would introduce counting-
   to-infinity behavior via two or more L1L2 routers, which would have a
   bad impact on network stability.

   The order of preference of IP routes in IS-IS is based on a few
   assumptions.

   - RFC 1195 defines that routes derived from L1 routing are preferred
     over routes derived from L2 routing.
   - The note in RFC 1195 paragraph 3.10.2, item 2c) defines that
     internal routes with internal metric-type and external prefixes
     with internal metric-type have the same preference.
   - RFC 1195 defines that external routes with internal metric-type are
     preferred over external routes with external metric type.
   - Routes derived from L2 routing are preferred over L2->L1 routes
     derived from L1 routing.

   Based on these assumptions, this document defines the following route
   preferences.

    1) L1 intra-area routes with internal metric
       L1 external routes with internal metric
    2) L2 intra-area routes with internal metric
       L2 external routes with internal metric
       L1->L2 inter-area routes with internal metric
       L1->L2 inter-area external routes with internal metric
    3) L2->L1 inter-area routes with internal metric
       L2->L1 inter-area external routes with internal metric
    4) L1 external routes with external metric
    5) L2 external routes with external metric
       L1->L2 inter-area external routes with external metric
    6) L2->L1 inter-area external routes with external metric

3.3 Additional notes on what prefixes to accept or advertise

   Paragraphs 4.1 and 4.2 enumerate all used IP route types in IS-IS.
   Besides these defined route types, the encoding used would allow for
   a few more potential combinations.  One of them is the combination of
   "IP Internal Reachability Information" and external metric type.
   This combination should never be used when building an LSP.  Upon
   receipt of an IP prefix with this combination, routers must ignore
   this prefix.

   Another issue would be the usage of the up/down bit in L2 LSPs.
   Because IS-IS is currently defined with two levels of hierarchy,
   there should never be a need to set the up/down bit in L2 LSPs.
   However, if IS-IS would ever be extended with more than two levels of
   hierarchy, L2-only (or L1L2) routers will need to be able to accept
   L2 IP routes with the up/down bit set.  Therefore, it is recommended
   that implementations ignore the up/down bit in L2 LSPs, and accept
   the prefixes in L2 LSPs regardless whether the up/down bit is set.
   This will allow for simpler migration once more than two levels of
   hierarchy are defined.

   Another detail that implementors should be aware of is the fact that
   L1L2 routers should only advertise in their L2 LSP those L1 routes
   that they use for forwarding themselves.  They should not
   unconditionally advertise into L2 all prefixes from LSPs in the L1
   database.

   Not all prefixes need to be advertised up or down the hierarchy.
   Implementations might allow for additional manual filtering or
   summarization to further bring down the number of inter-area prefixes
   they advertise in their LSPs.  It is also recommended that the
   default configuration of L1L2 routers is to not advertise any L2
   routes into L1 (see also paragraph 5.0).

4. Inter-operability with older implementations

   The solution in this document is not fully compatible with RFC 1195.
   It is an extension to RFC 1195.  If routers do not use the new
   functionality of external L1 routes, nor L2->L1 inter-area routes,
   older implementations that strictly follow RFC 1195 will be
   compatible with newer implementations that follow this document.

   Implementations that do not accept the "IP External Reachability
   Information" TLV in L1 LSPs will not be able to compute external L1
   routes.  This could cause routing loops between L1-only routers that
   do understand external L1 routes for a particular destination, and
   L1-only routers that use the default route pointing the closest
   attached L1L2 router for that destination.

   Implementations that follow RFC 1195 should ignore bit 8 in the
   default metric field when computing routes.  Therefore, even older
   implementations that do not know of the up/down bit should be able to
   accept the new L2->L1 inter-area routes.  These older implementations
   will install the new L2->L1 inter-area routes as L1 intra-area
   routes, but that in itself does not cause routing loops among L1-only
   routers.

   However, it is vital that the up/down bit is recognized by L1L2
   routers.  As has been stated before, L1L2 routers must never
   advertise L2->L1 inter-area routes back into L2.  Therefore, if L2
   routes are advertised down into L1 area, it is required that all L1L2
   routers in that area run software that understands the new up/down
   bit.  Older implementations that follow RFC 1195 and do not
   understand the new up/down bit will threat the L2->L1 inter-area
   routes as L1 intra-area routes, and they will advertise these routes
   back into L2.  This can cause routing loops, sub-optimal routing or
   extra routing instability.  For this reason it is recommended that

   implementations by default do not advertise any L2 routes into L1.
   Implementations should force the network administrator to manually
   configure L1L2 routers to advertise any L2 routes into L1.

5. Comparisons with other proposals

   In [3], a new TLV is defined to transport IP prefix information.
   This TLV format also defines an up/down bit to allow for L2->L1
   inter-area routes.  [3] also defines a new TLV to describe links.
   Both TLVs have wider metric space, and have the possibility to define
   sub-TLVs to advertise extra information belonging to the link or
   prefix.  The wider metric space in IP prefix TLVs allows for more
   granular metric information about inter-area path costs.  To make
   full use of the wider metric space, network administrators must
   deploy both new TLVs at the same time.

   Deployment of [3] requires an upgrade of all routers in the network
   and a transition to the new TLVs.  Such a network-wide upgrade and
   transition might not be an easy task.  In this case, the solution
   defined in this document, which requires only an upgrade of L1L2
   routers in selected areas, might be a good alternative to the
   solution defined in [3].

6. Security Considerations

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

7. References

   [1] ISO 10589, "Intermediate System to Intermediate System Intra-
       Domain Routing Exchange Protocol for use in Conjunction with the
       Protocol for Providing the Connectionless-mode Network Service
       (ISO 8473)".  [Also republished as RFC 1142.]

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

   [3] Smit, H. and T. Li, "IS-IS Extensions for Traffic Engineering",
       Work in Progress.

8. Authors' Addresses

   Tony Li
   Procket Networks
   1100 Cadillac Court
   Milpitas, CA 95035-3025

   EMail: tli@procket.com


   Tony Przygienda
   Redback
   350 Holger Way
   San Jose, CA 95134

   EMail: prz@redback.com


   Henk Smit
   Procket Networks
   1100 Cadillac Court
   Milpitas, CA 95035-3025

   EMail: henk@procket.com

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