rfc2966.txt

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   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.



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   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.



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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.










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   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.



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   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.



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   - 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