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rfc:rfc3345

Network Working Group D. McPherson Request for Comments: 3345 TCB Category: Informational V. Gill

                                                 AOL Time Warner, Inc.
                                                             D. Walton
                                                             A. Retana
                                                   Cisco Systems, Inc.
                                                           August 2002
Border Gateway Protocol (BGP) Persistent Route Oscillation Condition

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 (2002).  All Rights Reserved.

Abstract

 In particular configurations, the BGP scaling mechanisms defined in
 "BGP Route Reflection - An Alternative to Full Mesh IBGP" and
 "Autonomous System Confederations for BGP" will introduce persistent
 BGP route oscillation.  This document discusses the two types of
 persistent route oscillation that have been identified, describes
 when these conditions will occur, and provides some network design
 guidelines to avoid introducing such occurrences.

1. Introduction

 The Border Gateway Protocol (BGP) is an inter-Autonomous System
 routing protocol.  The primary function of a BGP speaking system is
 to exchange network reachability information with other BGP systems.
 In particular configurations, the BGP [1] scaling mechanisms defined
 in "BGP Route Reflection - An Alternative to Full Mesh IBGP" [2] and
 "Autonomous System Confederations for BGP" [3] will introduce
 persistent BGP route oscillation.
 The problem is inherent in the way BGP works: locally defined routing
 policies may conflict globally, and certain types of conflicts can
 cause persistent oscillation of the protocol.  Given current
 practices, we happen to see the problem manifest itself in the
 context of MED + route reflectors or confederations.

McPherson, et al. Informational [Page 1] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

 The current specification of BGP-4 [4] states that the
 MULTI_EXIT_DISC is only comparable between routes learned from the
 same neighboring AS.  This limitation is consistent with the
 description of the attribute: "The MULTI_EXIT_DISC attribute may be
 used on external (inter-AS) links to discriminate among multiple exit
 or entry points to the same neighboring AS." [1,4]
 In a full mesh iBGP network, all the internal routers have complete
 visibility of the available exit points into a neighboring AS.  The
 comparison of the MULTI_EXIT_DISC for only some paths is not a
 problem.
 Because of the scalability implications of a full mesh iBGP network,
 two alternatives have been standardized: route reflectors [2] and AS
 confederations [3].  Both alternatives describe methods by which
 route distribution may be achieved without a full iBGP mesh in an AS.
 The route reflector alternative defines the ability to re-advertise
 (reflect) iBGP-learned routes to other iBGP peers once the best path
 is selected [2].  AS Confederations specify the operation of a
 collection of autonomous systems under a common administration as a
 single entity (i.e. from the outside, the internal topology and the
 existence of separate autonomous systems are not visible).  In both
 cases, the reduction of the iBGP full mesh results in the fact that
 not all the BGP speakers in the AS have complete visibility of the
 available exit points into a neighboring AS.  In fact, the visibility
 may be partial and inconsistent depending on the location (and
 function) of the router in the AS.
 In certain topologies involving either route reflectors or
 confederations (detailed description later in this document), the
 partial visibility of the available exit points into a neighboring AS
 may result in an inconsistent best path selection decision as the
 routers don't have all the relevant information.  If the
 inconsistencies span more than one peering router, they may result in
 a persistent route oscillation.  The best path selection rules
 applied in this document are consistent with the current
 specification [4].
 The persistent route oscillation behavior is deterministic and can be
 avoided by employing some rudimentary BGP network design principles
 until protocol enhancements resolve the problem.
 In the following sections a taxonomy of the types of oscillations is
 presented and a description of the set of conditions that will
 trigger route oscillations is given.  We continue by providing
 several network design alternatives that remove the potential of this
 occurrence.

McPherson, et al. Informational [Page 2] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

 It is the intent of the authors that this document serve to increase
 operator awareness of the problem, as well as to trigger discussion
 and subsequent proposals for potential protocol enhancements that
 remove the possibility of this to occur.
 The oscillations are classified into Type I and Type II depending
 upon the criteria documented below.

2. Discussion of Type I Churn

 In the following two subsections we provide configurations under
 which Type I Churn will occur.  We begin with a discussion of the
 problem when using Route Reflection, and then discuss the problem as
 it relates to AS Confederations.
 In general, Type I Churn occurs only when BOTH of the following
 conditions are met:
    1) a single-level Route Reflection or AS Confederations design is
       used in the network AND
    2) the network accepts the BGP MULTI_EXIT_DISC (MED) attribute
       from two or more ASs for a single prefix and the MED values are
       unique.
 It is also possible for the non-deterministic ordering of paths to
 cause the route oscillation problem.  [1] does not specify that paths
 should be ordered based on MEDs but it has been proven that non-
 deterministic ordering can lead to loops and inconsistent routing
 decisions.  Most vendors have either implemented deterministic
 ordering as default behavior, or provide a knob that permits the
 operator to configure the router to order paths in a deterministic
 manner based on MEDs.

McPherson, et al. Informational [Page 3] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

2.1. Route Reflection and Type I Churn

 We now discuss Type I oscillation as it relates to Route Reflection.
 To begin, consider the topology depicted in Figure 1:
  1. ————————————————————–

/ ——————– ——————– \

 |    /                      \           /                      \    |
 |   |       Cluster 1        |         |      Cluster 2         |   |
 |   |                        |         |                        |   |
 |   |                        |   *1    |                        |   |
 |   |         Ra(RR) . . . . . . . . . . . . . . Rd(RR)         |   |
 |   |         .  .           |         |           .            |   |
 |   |       .*5    .*4       |         |           .*12         |   |
 |   |     .          .       |         |           .            |   |
 |   |   Rb(C)        Rc(C)   |         |         Re(C)          |   |
 |   |     .            .     |         |           .            |   |
 |    \    .            .    /           \          .           /    |
 |      ---.------------.---               ---------.----------      |
  \        .(10)        .(1)     AS1                .(0)            /
    -------.------------.---------------------------.--------------
           .            .                           .
        ------            .     ------------      .
      /        \            . /              \   .
     |   AS10   |            |      AS6       |
      \        /              \              /
        ------                  ------------
              .                      .
                 .                   .
                    .       --------------
                       .  /                \
                         |      AS100       |- 10.0.0.0/8
                          \                /
                            --------------
           Figure 1: Example Route Reflection Topology
 In Figure 1 AS1 contains two Route Reflector Clusters, Clusters 1 and
 2.  Each Cluster contains one Route Reflector (RR) (i.e., Ra and Rd,
 respectively).  An associated 'RR' in parentheses represents each RR.
 Cluster 1 contains two RR Clients (Rb and Rc), and Cluster 2 contains
 one RR Client (Re).  An associated 'C' in parentheses indicates RR
 Client status.  The dotted lines are used to represent BGP peering
 sessions.
 The number contained in parentheses on the AS1 EBGP peering sessions
 represents the MED value advertised by the peer to be associated with
 the 10.0.0.0/8 network reachability advertisement.

McPherson, et al. Informational [Page 4] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

 The number following each '*' on the IBGP peering sessions represents
 the additive IGP metrics that are to be associated with the BGP
 NEXT_HOP attribute for the concerned route.  For example, the Ra IGP
 metric value associated with a NEXT_HOP learned via Rb would be 5;
 while the metric value associated with the NEXT_HOP learned via Re
 would be 13.
 Table 1 depicts the 10.0.0.0/8 route attributes as seen by routers
 Rb, Rc and Re, respectively.  Note that the IGP metrics in Figure 1
 are only of concern when advertising the route to an IBGP peer.
          Router  MED  AS_PATH
          --------------------
          Rb       10   10 100
          Rc        1    6 100
          Re        0    6 100
          Table 1: Route Attribute Table
 For the following steps 1 through 5, the best path will be marked
 with a '*'.
    1) Ra has the following installed in its BGP table, with the path
       learned via AS2 marked best:
                          NEXT_HOP
           AS_PATH  MED   IGP Cost
           -----------------------
             6 100    1          4
          * 10 100   10          5
       The '10 100' route should not be marked as best, though this is
       not the cause of the persistent route oscillation.  Ra realizes
       it has the wrong route marked as best since the '6 100' path
       has a lower IGP metric.  As such, Ra makes this change and
       advertises an UPDATE message to its neighbors to let them know
       that it now considers the '6 100, 1, 4' route as best.
    2) Rd receives the UPDATE from Ra, which leaves Rd with the
       following installed in its BGP table:
                          NEXT_HOP
           AS_PATH  MED   IGP Cost
           -----------------------
          *  6 100    0         12
             6 100    1          5

McPherson, et al. Informational [Page 5] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

       Rd then marks the '6 100, 0, 12' route as best because it has a
       lower MED.  Rd sends an UPDATE message to its neighbors to let
       them know that this is the best route.
    3) Ra receives the UPDATE message from Rd and now has the
       following in its BGP table:
                          NEXT_HOP
           AS_PATH  MED   IGP Cost
           -----------------------
             6 100    0         13
             6 100    1          4
          * 10 100   10          5
       The first route (6 100, 0, 13) beats the second route (6 100,
       1, 4) because of a lower MED.  Then the third route (10 100,
       10, 5) beats the first route because of lower IGP metric to
       NEXT_HOP.  Ra sends an UPDATE message to its peers informing
       them of the new best route.
    4) Rd receives the UPDATE message from Ra, which leaves Rd with
       the following BGP table:
                          NEXT_HOP
           AS_PATH  MED   IGP Cost
           -----------------------
             6 100    0         12
          * 10 100   10          6
       Rd selects the '10 100, 10, 6' path as best because of the IGP
       metric.  Rd sends an UPDATE/withdraw to its peers letting them
       know this is the best route.
    5) Ra receives the UPDATE message from Rd, which leaves Ra with
       the following BGP table:
                          NEXT_HOP
           AS_PATH  MED   IGP Cost
           -----------------------
             6 100    1          4
          * 10 100   10          5
       Ra received an UPDATE/withdraw for '6 100, 0, 13', which
       changes what is considered the best route for Ra.  This is why
       Ra has the '10 100, 10, 5' route selected as best in Step 1,
       even though '6 100, 1, 4' is actually better.

McPherson, et al. Informational [Page 6] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

    At this point, we've made a full loop and are back at Step 1.  The
    router realizes it is using the incorrect best path, and repeats
    the cycle.  This is an example of Type I Churn when using Route
    Reflection.

2.2. AS Confederations and Type I Churn

 Now we provide an example of Type I Churn occurring with AS
 Confederations.  To begin, consider the topology depicted in Figure
 2:
  1. ————————————————————–

/ ——————– ——————– \

|    /                      \           /                      \    |
|   |       Sub-AS 65000     |         |      Sub-AS 65001      |   |
|   |                        |         |                        |   |
|   |                        |   *1    |                        |   |
|   |         Ra . . . . . . . . . . . . . . . . . Rd           |   |
|   |         .  .           |         |           .            |   |
|   |       .*3    .*2       |         |           .*6          |   |
|   |     .          .       |         |           .            |   |
|   |    Rb . . . . . Rc     |         |          Re            |   |
|   |     .    *5      .     |         |           .            |   |
|    \    .            .    /           \          .           /    |
|      ---.------------.---               ---------.----------      |
 \        .(10)        .(1)     AS1                .(0)            /
   -------.------------.---------------------------.--------------
          .            .                           .
       ------            .     ------------      .
     /        \            . /              \  .
    |   AS10   |            |      AS6       |
     \        /              \              /
       ------                  ------------
             .                      .
                .                   .
                   .       --------------
                      .  /                \
                        |      AS100       |- 10.0.0.0/8
                         \                /
                           --------------
          Figure 2: Example AS Confederations Topology
 The number contained in parentheses on each AS1 EBGP peering session
 represents the MED value advertised by the peer to be associated with
 the 10.0.0.0/8 network reachability advertisement.

McPherson, et al. Informational [Page 7] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

 The number following each '*' on the IBGP peering sessions represents
 the additive IGP metrics that are to be associated with the BGP
 NEXT_HOP attribute for the concerned route.
 For example, the Ra IGP metric value associated with a NEXT_HOP
 learned via Rb would be 3; while the metric value associated with the
 NEXT_HOP learned via Re would be 6.
 Table 2 depicts the 10.0.0.0/8 route attributes as seen by routers
 Rb, Rc and Re, respectively.  Note that the IGP metrics in Figure 2
 are only of concern when advertising the route to an IBGP peer.
       Router  MED  AS_PATH
       --------------------
       Rb       10   10 100
       Rc        1    6 100
       Re        0    6 100
       Table 2: Route Attribute Table
 For the following steps 1 through 6 the best route will be marked
 with an '*'.
    1) Ra has the following BGP table:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
          *         10 100   10          3
             (65001) 6 100    0          7
                     6 100    1          2
       The '10 100' route is selected as best and is advertised to Rd,
       though this is not the cause of the persistent route
       oscillation.
    2) Rd has the following in its BGP table:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
                     6 100    0          6
          * (65000) 10 100   10          4
       The '(65000) 10 100' route is selected as best because it has
       the lowest IGP metric.  As a result, Rd sends an
       UPDATE/withdraw to Ra for the '6 100' route that it had
       previously advertised.

McPherson, et al. Informational [Page 8] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

    3) Ra receives the withdraw from Rd.  Ra now has the following in
       its BGP table:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
          *         10 100   10          3
                     6 100    1          2
       Ra received a withdraw for '(65001) 6 100', which changes what
       is considered the best route for Ra.  Ra does not compute the
       best path for a prefix unless its best route was withdrawn.
       This is why Ra has the '10 100, 10, 3' route selected as best,
       even though the '6 100, 1, 2' route is better.
    4) Ra's periodic BGP scanner runs and realizes that the '6 100'
       route is better because of the lower IGP metric.  Ra sends an
       UPDATE/withdraw to Rd for the '10 100' route since Ra is now
       using the '6 100' path as its best route.
       Ra's BGP table looks like this:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
                    10 100   10          3
          *          6 100    1          2
    5) Rd receives the UPDATE from Ra and now has the following in its
       BGP table:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
             (65000) 6 100    1          3
          *          6 100    0          6
       Rd selects the '6 100, 0, 6' route as best because of the lower
       MED value.  Rd sends an UPDATE message to Ra, reporting that '6
       100, 0, 6' is now the best route.

McPherson, et al. Informational [Page 9] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

    6) Ra receives the UPDATE from Rd.  Ra now has the following in
       its BGP table:
                                  NEXT_HOP
                   AS_PATH  MED   IGP Cost
           -------------------------------
          *         10 100   10          3
             (65001) 6 100    0          7
                     6 100    1          2
       At this point we have made a full cycle and are back to step 1.
       This is an example of Type I Churn with AS Confederations.

2.3. Potential Workarounds for Type I Churn

 There are a number of alternatives that can be employed to avoid this
 problem:
    1) When using Route Reflection make sure that the inter-Cluster
       links have a higher IGP metric than the intra-Cluster links.
       This is the preferred choice when using Route Reflection.  Had
       the inter-Cluster IGP metrics been much larger than the intra-
       Cluster IGP metrics, the above would not have occurred.
    2) When using AS Confederations ensure that the inter-Sub-AS links
       have a higher IGP metric than the intra-Sub-AS links.  This is
       the preferred option when using AS Confederations.  Had the
       inter-Sub-AS IGP metrics been much larger than the intra-Sub-AS
       IGP metrics, the above would not have occurred.
    3) Do not accept MEDs from peers (this may not be a feasible
       alternative).
    4) Utilize other BGP attributes higher in the decision process so
       that the BGP decision algorithm never reaches the MED step.  As
       using this completely overrides MEDs, Option 3 may make more
       sense.
    5) Always compare BGP MEDs, regardless of whether or not they were
       obtained from a single AS.  This is probably a bad idea since
       MEDs may be derived in a number of ways, and are typically done
       so as a matter of operator-specific policy.  As such, comparing
       MED values for a single prefix learned from multiple ASs is
       ill-advised.  Of course, this mostly defeats the purpose of
       MEDs, and as such, Option 3 may be a more viable alternative.
    6) Use a full IBGP mesh.  This is not a feasible solution for ASs
       with a large number of BGP speakers.

McPherson, et al. Informational [Page 10] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

3. Discussion of Type II Churn

 In the following subsection we provide configurations under which
 Type II Churn will occur when using AS Confederations.  For the sake
 of brevity, we avoid similar discussion of the occurrence when using
 Route Reflection.
 In general, Type II churn occurs only when BOTH of the following
 conditions are met:
    1) More than one tier of Route Reflection or Sub-ASs is used in
       the network AND
    2) the network accepts the BGP MULTI_EXIT_DISC (MED) attribute
       from two or more ASs for a single prefix and the MED values are
       unique.

McPherson, et al. Informational [Page 11] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

3.1. AS Confederations and Type II Churn

 Let's now examine the occurrence of Type II Churn as it relates to AS
 Confederations.  Figure 3 provides our sample topology:
  1. ————————————————————–

/ ——————- \

|      AS 1          /      Sub-AS 65500   \                         |
|                   |                       |                        |
|                   |    Rc . . . . Rd      |                        |
|                   |    .   *2      .      |                        |
|                    \  .              .   /                         |
|                      .-----------------.                           |
|                     .*40                 .*40                      |
|      --------------.-----                --.-----------------      |
|    /              .        \           /     .                \    |
|   |   Sub-AS     .          |         |        .      Sub-AS   |   |
|   |    65501    .           |         |          .     65502   |   |
|   |          Rb             |         |         Re             |   |
|   |          .              |         |        . .             |   |
|   |          .*10           |         |     *2.   .*3          |   |
|   |          .              |         |      .     .           |   |
|   |          Ra             |         |  . Rg . . . Rf         |   |
|    \          .            /           .             .        /    |
|      ----------.----------           .  -------------.-------      |
 \                .(0)               .(1)              .()          /
   ----------------.---------------.-------------------.----------
                   .            .                     .
                    ---------  .                  ---------
                    |AS 200 |                     |AS 300 |
                    ---------                     ---------
                            .                     .
                              .                 .
                              -------------------
                              |      AS 400     | - 10.0.0.0/8
                              -------------------
          Figure 3: Example AS Confederations Topology
 In Figure 3 AS 1 contains three Sub-ASs, 65500, 65501 and 65502.  No
 RR is used within the Sub-AS, and as such, all routers within each
 Sub-AS are fully meshed.  Ra and Rb are members of Sub-AS 65501.  Rc
 and Rd are members of Sub-AS 65500.  Ra and Rg are EBGP peering with
 AS 200, router Rf has an EBGP peering with AS 300.  AS 200 and AS 300
 provide transit for AS 400, and in particular, the 10/8 network.  The
 dotted lines are used to represent BGP peering sessions.

McPherson, et al. Informational [Page 12] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

 The number following each '*' on the BGP peering sessions represents
 the additive IGP metrics that are to be associated with the BGP
 NEXT_HOP.  The number contained in parentheses on each AS 1 EBGP
 peering session represents the MED value advertised by the peer to be
 associated with the network reachability advertisement (10.0.0.0/8).
 Rc, Rd and Re are the primary routers involved in the churn, and as
 such, will be the only BGP tables that we will monitor step by step.
 For the following steps 1 through 8 each router's best route will be
 marked with a '*'.
    1) Re receives the AS 400 10.0.0.0/8 route advertisement via AS
       200 from Rg and AS 300 from Rf.  Re selects the path via Rg and
       AS 200 because of IGP metric (Re didn't consider MED because
       the advertisements were received from different ASs).
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Re   * 200 400    1          2
                 300 400               3
       Re sends an UPDATE message to Rd advertising its new best path
       '200 400, 1'.
    2) The '200 400, 0' path was advertised from Ra to Rb, and then
       from Rb to Rc.  Rd learns the '200 400, 1' path from Re.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          -------------------------------
          Rc   * 200 400   0         50
          Rd   * 200 400   1         42
          Re     300 400              3
               * 200 400   1          2

McPherson, et al. Informational [Page 13] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

    3) Rc and Rd advertise their best paths to each other; Rd selects
       '200 400, 0' because of the MED.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc   * 200 400   0         50
                 200 400   1         44
          Rd   * 200 400   0         52
                 200 400   1         42
          Re     300 400              3
               * 200 400   1          2
       Rd has a new best path so it sends an UPDATE to to Re,
       announcing the new path and an UPDATE/withdraw for '200 400, 1'
       to Rc.
    4) Re now selects '300 400' (with no MED) because '200 400, 0'
       beats '200 400, 1' based on MED and '300 400' beats '200 400,
       0' because of IGP metric.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc   * 200 400    0         50
          Rd   * 200 400    0         52
                 200 400    1         42
          Re   * 300 400               3
                 200 400    0         92
       Re has a new best path and sends an UPDATE to Rd for '300 400'.
 5) Rd selects the '300 400' path because of IGP metric.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc   * 200 400    0         50
          Rd     200 400    0         52
               * 300 400              43
          Re   * 300 400               3
                 200 400    0         92
                 200 400    1          2
       Rd has a new best path so it sends an UPDATE to Rc and a
       UPDATE/withdraw to Re for '200 400, 0'.

McPherson, et al. Informational [Page 14] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

    6) Rc selects '300 400' because of the IGP metric.  Re selects
       '200 400, 1' because of the IGP metric.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc     200 400    0         50
               * 300 400              45
          Rd     200 400    0         52
               * 300 400              43
          Re     300 400               3
               * 200 400    1          2
       Rc sends an UPDATE/withdraw for '200 400, 0' to Rd.  Re sends
       an UPDATE for '200 400, 1' to Rd.
    7) Rd selects '200 400, 1' as its new best path based on the IGP
       metric.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc     200 400    0         50
               * 300 400              45
          Rd   * 200 400    1         42
          Re     300 400               3
               * 200 400    1          2
       Rd sends an UPDATE to Rc, announcing '200 400, 1' and
       implicitly withdraws '300 400'.
    8) Rc selects '200 400, 0'.
                                NEXT_HOP
          Router AS_PATH  MED   IGP Cost
          ------------------------------
          Rc   * 200 400    0         50
                 200 400    1         44
          Rd   * 200 400    1         42
          Re     300 400               3
               * 200 400    1          2
       At this point we are back to Step 2 and are in a loop.

McPherson, et al. Informational [Page 15] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

3.2. Potential Workarounds for Type II Churn

 1) Do not accept MEDs from peers (this may not be a feasible
    alternative).
 2) Utilize other BGP attributes higher in the decision process so
    that the BGP decision algorithm selects a single AS before it
    reaches the MED step.  For example, if local-pref were set based
    on the advertising AS, then you first eliminate all routes except
    those in a single AS.  In the example, router Re would pick either
    X or Y based on your local-pref and never change selections.
    This leaves two simple workarounds for the two types of problems.
    Type I:  Make inter-cluster or inter-sub-AS link metrics higher
    than intra-cluster or intra-sub-AS metrics.
    Type II: Make route selections based on local-pref assigned to the
    advertising AS first and then use IGP cost and MED to make
    selection among routes from the same AS.
    Note that this requires per-prefix policies, as well as near
    intimate knowledge of other networks by the network operator.  The
    authors are not aware of ANY [large] provider today that performs
    per-prefix policies on routes learned from peers.  Implicitly
    removing this dynamic portion of route selection does not appear
    to be a viable option in today's networks.  The main point is that
    an available workaround using local-pref so that no two AS's
    advertise a given prefix at the same local-pref solves type II
    churn.
 3) Always compare BGP MEDs, regardless of whether or not they were
    obtained from a single AS.  This is probably a bad idea since MEDs
    may be derived in a number of ways, and are typically done so as a
    matter of operator-specific policy and largely a function of
    available metric space provided by the employed IGP.  As such,
    comparing MED values for a single prefix learned from multiple ASs
    is ill-advised.  This mostly defeats the purpose of MEDs; Option 1
    may be a more viable alternative.
 4) Do not use more than one tier of Route Reflection or Sub-ASs in
    the network.   The risk of route oscillation should be considered
    when designing networks that might use a multi-tiered routing
    isolation architecture.
 5) In a RR topology, mesh the clients.  For confederations, mesh the
    border routers at each level in the hierarchy.  In Figure 3, for
    example, if Rb and Re are peers, then there's no churn.

McPherson, et al. Informational [Page 16] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

4. Future Work

 It should be stated that protocol enhancements regarding this problem
 must be pursued.  Imposing network design requirements, such as those
 outlined above, are clearly an unreasonable long-term solution.
 Problems such as this should not occur under 'default' protocol
 configurations.

5. Security Considerations

 This discussion introduces no new security concerns to BGP or other
 specifications referenced in this document.

6. Acknowledgments

 The authors would like to thank Curtis Villamizar, Tim Griffin, John
 Scudder, Ron Da Silva, Jeffrey Haas and Bill Fenner.

7. References

 [1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
     1771, March 1995.
 [2] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection - An
     Alternative to Full Mesh IBGP", RFC 2796, April 2000.
 [3] Traina, P., McPherson, D. and J. Scudder, J., "Autonomous System
     Confederations for BGP", RFC 3065, February 2001.
 [4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
     Work in Progress.

McPherson, et al. Informational [Page 17] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

8. Authors' Addresses

 Danny McPherson
 TCB
 EMail: danny@tcb.net
 Vijay Gill
 AOL Time Warner, Inc.
 12100 Sunrise Valley Drive
 Reston, VA 20191
 EMail: vijay@umbc.edu
 Daniel Walton
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 EMail: dwalton@cisco.com
 Alvaro Retana
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 EMail: aretana@cisco.com

McPherson, et al. Informational [Page 18] RFC 3345 BGP Persistent Route Oscillation Condition August 2002

9. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  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
 developing 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

McPherson, et al. Informational [Page 19]

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