Network Working Group                                         P. Marques
Expires: October 22, 2011 July 6, 2012                                        R. Fernando
                                                                 E. Chen
                                                            P. Mohapatra
                                                           Cisco Systems
                                                              H. Gredler
                                                        Juniper Networks
                                                          April 20, 2011
                                                         January 3, 2012

            Advertisement of the best external route in BGP


   The current BGP-4 protocol specification [RFC4271] states that the
   selection process chooses the best path for a given route which is
   added to the Loc-Rib and advertised to all peers.

   Previous versions [RFC1771] of the specification defined a different
   rule for Internal BGP Updates.  Given that Internal paths are not re-
   advertised to Internal peers, it was specified that the best of the
   external paths, as determined by the path selection tie breaking
   algorithm, would be advertised to Internal peers.

   This document extends that procedure to operate in environments where
   Route Reflection [RFC4456] or Confederations [RFC5065] are used and
   explains why advertising the additional routing information can
   improve convergence time without causing routing loops.

   Additional benefits include reduction of inter-domain churn and
   avoidance of permanent route oscillation.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 22, 2011. July 6, 2012.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Generalization . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Algorithm for selection of the Adj-RIB-OUT path  . . . . . . .  7
   5.  Advertisement Rules  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Consistency between routing and forwarding . . . . . . . . . . 10
   7.  Applications . . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Fast Connectivity Restoration  . . . . . . . . . . . . . . . . 13
   9.  Inter-Domain Churn Reduction . . . . . . . . . . . . . . . . . 14
   10. Reducing Persistent IBGP oscillation . . . . . . . . . . . . . 15
   11. Deployment Considerations  . . . . . . . . . . . . . . . . . . 16
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

1.  Introduction

   Earlier versions of the BGP-4 protocol specification [RFC1771]
   prescribed different route advertisement rules for Internal and
   External peers.  While the overall best path would be advertised to
   External peers, Internal peers are advertised the best of the
   externally received paths.

   This Internal advertisement rule was never implemented as specified
   and was latter dropped from the protocol.  There is a trade-off in
   advertising the "best-external" route versus the behavior that became
   common standard of not advertising the route when the selected best
   path is received from an Internal peer.  By not advertising
   information in this case it is possible to reduce state both in the
   local BGP speaker as well as in the network overall.  Early BGP
   implementations where very concerned with reducing state as they
   where limited to relatively low memory footprints (e.g. 16 MB).
   There is also the possible concern regarding advertising a path
   different than the path that has been selected for forwarding.

   However, advertising the best external route, when different from the
   best route, presents additional information into an IBGP mesh which
   may be of value for several purposes including:

   o  Faster restoration of connectivity.  By providing additional
      paths, that may be used to fail over in case the primary path
      becomes invalid or is withdrawn.

   o  Reducing inter-domain churn and traffic black-holing due to the
      readily available alternate path.

   o  Reducing the potential for situations of permanent IBGP route
      oscillation [RFC3345].

   o  Improving selection of lower MED routes from the same neighboring

   This document defines procedures to select the best external route
   for each peer.  It also describes how above benefits are realized
   with best external route announcement with the help of certain

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Generalization

   The BGP-4 protocol [RFC1771] has extended with two alternative
   mechanisms that provide ways to reduce the operational complexity of
   route distribution within an AS: Route Reflection [RFC4456] and
   Confederations [RFC5065].  It is important to be able to express
   route advertisement rules in the context of both of these mechanisms.

   When Route Reflection is used, Internal peers are further classified
   depending of the reflection cluster they belong to.  Non-client
   internal peers form one BGP peering mesh.  Each set of RR clients
   with the same "cluster-id" configuration forms a separate mesh.

   When selecting the path to add to the Adj-RIB-OUT, this document
   specifies that the path that originate from the same mesh MAY be
   excluded from consideration.  This results in an Adj-RIB-OUT
   selection per mesh (the set of non-client peers or a specific

   Similarly, when BGP Confederations are used, each confederated AS is
   a BGP mesh.  As with the Route Reflection scenario, when selecting
   the path to add to the Adj-RIB-OUT, routes from the same mesh MAY be

4.  Algorithm for selection of the Adj-RIB-OUT path

   The objective of this protocol extension is to improve the quality of
   the routing information known to a particular BGP mesh with minimum
   additional cost in terms of processing and state.

   Towards that goal, it is useful to define a total order between the
   Adj-RIB-In routes which provides both the same overall best path as
   the algorithm defined in the current BGP-4 specification [RFC4271] as
   well as an ordering of alternate routes.  Using this total order it
   is then computationally efficient to select the path for a specific
   Adj-RIB-OUT by excluding the routes that have been received from the
   BGP mesh corresponding to the peer (or set of peers).

   In order to achieve this, it is helpful to introduce the concept of
   path group.  A group is the set of paths that compare as equal
   through all the steps prior to the MED comparison step (as defined in
   section of RFC 4271 [RFC4271] and have been received from the
   same neighbor AS.

   Paths are ordered within a group via MED or subsequent route
   selection rules.

   In pseudo-code:

      function compare(path_1, path_2) {
          cmp_result cmp = selection_steps_before_med(path_1, path_2);
          if (cmp != cmp_result.equal) {
              return cmp;
          if (neighbor_as(path_1) == neighbor_as(path_2)) {
              return selection_steps_after_med(path_1, path_2);

          if (is_group_best(path_1)) {
              if (!is_group_best(path_2)) {
                  return cmp_result.greater_than;
              return selection_steps_after_med(path_1, path_2);
          } else {
              if (is_group_best(path_2)) {
                  return cmp_result.less_than;
              /* Compare the best paths of respective groups */
              return compare(group_best(path_1), group_best(path_2));

   As an example, the following set of received routes:

                       | Path | AS | MED | rtr_id |
                       | a    | 1  | 10  | 10     |
                       |      |    |     |        |
                       | b    | 2  | 5   | 1      |
                       |      |    |     |        |
                       | c    | 1  | 5   | 5      |
                       |      |    |     |        |
                       | d    | 2  | 20  | 20     |
                       |      |    |     |        |
                       | e    | 2  | 30  | 30     |
                       |      |    |     |        |
                       | f    | 3  | 10  | 20     |

                           Path Attribute Table

   Would yield the following order (from the most to the least

      b < d < e < c < a < f

   In this example, comparison of the best path within each group
   provides the sequence (b < c < f).  The remaining paths are ordered
   in relation to their respective group best.

   The first path in the ordering above is the best overall path for a
   given NLRI.  When selecting a path for a particular Adj-RIB-Out (or
   set of RIB-Outs) an implementation MAY choose to select the first
   path in the global order which was not received from the same BGP
   mesh (as defined above) as the target peer (or peers).

5.  Advertisement Rules

   1.  When advertising a route to a non-client Internal peer, a BGP
       speaker MAY choose to select the first path in order that did not
       originate from the same BGP mesh (i.e. the set of non-client
       Internal peers) whenever the best overall path has been received
       from this mesh and would be suppressed by the Internal BGP non-
       readvertisement rule.

   2.  When advertising a route to a Route Reflection client peer, in
       case the overall best path has been received from the same
       cluster, a BGP speaker MUST be able to advertise the best overall
       path to all the members of the cluster other than the originator,
       unless "client-to-client" reflection is disabled.  The
       implementation MAY choose to advertise an alternate path to the
       specific peer that originates the best overall path by excluding
       from consideration all paths with the same originator-id.

   3.  When "client-to-client" reflection is disabled and the cluster is
       operating as a mesh, a Route Reflector MAY opt to advertise to
       the cluster the preferred path from the set of paths not received
       from the cluster.  While this deployment mode is currently
       uncommon, it can be a practical way to guarantee path diversity
       inside the cluster.

   4.  A confederation border route MAY choose to advertise an alternate
       path towards its Internal BGP mesh or towards a con-fed member AS
       following the same procedure as defined above.

6.  Consistency between routing and forwarding

   The internal update advertisement rules contained in the original
   BGP-4 specification [RFC1771] can lead to situations where traffic is
   forwarded through a route other than the route advertised by BGP.

   Inconsistencies between forwarding and routing are highly
   undesirable.  Service providers use BGP with the dual objective of
   learning reach-ability information and expressing policy over network
   resources.  The latter assumes that forwarding follows routing

   Consider the Autonomous system presented in figure 1, where r1 ... r4
   are members of a single IBGP mesh and routes a, b, and c are received
   from external peers.

                    AS 1 (c)
                    +----+           +----+
                    | r1 |...........| r2 |
                    +----+           +----+
                    +----+           +----+
                    | r3 |...........| r4 | --- ebgp --- AS X
                    +----+           +----+
                   /      \
                  /        \
               AS 1 (a)  AS 2 (b)

                         Inconsistency in Routing

                       | Path | AS | MED | rte_id |
                       | a    | 1  | 10  | 1      |
                       |      |    |     |        |
                       | b    | 2  | 5   | 10     |
                       |      |    |     |        |
                       | c    | 1  | 5   | 5      |

                           Path Attribute Table

   Following the rules as specified in RFC 1771 [RFC1771], router r3
   will select path (b) received from AS 2 as its overall best to
   install in the Loc-Rib, since path (b) is preferable to path (c), the
   lowest MED route from AS 1.  However for the purposes of Internal
   Update route selection, it will ignore the presence of path (c), and
   elect (a) as its advertisement, via the router-id tie-breaking rule.

   In this scenario, router r4 will receive (c) from r1 and (a) from r3.
   It will pick the lowest MED route (c) and advertise it out via IBGP
   to AS X. However at this point routing is inconsistent with
   forwarding as traffic received from AS X will be forwarded towards AS
   2, while the IBGP advertisement is being made for an AS 1 path.

   Routing policies are typically specified in terms of neighboring
   AS-es.  In the situation above, assuming that AS 1 is network for
   which this AS provides transit services while AS 2 and AS X are peer
   networks, one can easily see how the inconsistency between routing
   and forwarding would lead to transit being inadvertently provided
   between AS X and AS 2.  This could lead to persistent forwarding

   Inconsistency between routing and forwarding may happen, whenever a
   GP speaker chooses to advertise an external route into IBGP that is
   different from the overall best route and its overall best is

7.  Applications
8.  Fast Connectivity Restoration

   When two exits are available to reach a particular destination and
   one is preferred over the other, the availability of an alternate
   path provides fast connectivity restoration when the primary path

   Restoration can be quick since the alternate path is already at hand.
   The border router could recompute the backup route and perinatal it
   in FIB ready to be switched when the primary goes away.  Note that
   this requires the border router that's the backup to also perinatal
   the secondary path and switch to it on failure.

9.  Inter-Domain Churn Reduction

   Within an AS, the non availability of backup best leads to a border
   router sending a withdraw upstream when the primary fails.  This
   leads to inter-domain churn and packet loss for the time the network
   takes to converge to the alternate path.  Having the alternate path
   will reduces the churn and eliminates packet loss.

10.  Reducing Persistent IBGP oscillation

   Advertising the best external route, according to the algorithm
   described in this document will reduce the possibility of route
   oscillation by introducing additional information into the IBGP

   For a permanent oscillation condition to occur, it is necessary that
   a circular dependency between paths occurs such that the selection of
   a new best path by a router, in response to a received IBGP
   advertisement, causes the withdrawal of information that another
   router depends on in order to generate the original event.

   In vanilla BGP, when only the best overall route is advertised, as in
   most implementations, oscillation can occur whenever there are 2 or
   clusters/sub-AS-es such that at least one cluster has more than one
   path that can potentially contribute to the dependency.

11.  Deployment Considerations

   The mechanism specified in the draft allows a BGP speaker to
   advertise a route that is not the best route used for forwarding.
   This is a departure from the current behavior.  However, consistency
   in the path selection process across the AS is still guaranteed since
   the ingress routers will not choose the best-external route as the
   best route for a destination in steady state (for the same reason
   that the BGP speaker announcing the best-external route chose an IBGP
   route as best instead of the externally learnt route).  Though it is
   possible to alter this assurance by defining route policies on IBGP
   sessions, use of such policies in IBGP is not recommended, especially
   with best-external announcement turned on in the network.  It is also
   worth noting that such inconsistency in routing and forwarding is
   mitigated in a tunneled network.

12.  Acknowledgments

   This document greatly benefits from the comments of Yakov Rekhter,
   John Scudder, Eric Rosen, Jenny Yuan, Jay Borkenhagen, Salkat Ray and
   Jakob Heitz.

13.  IANA Considerations

   This document has no actions for IANA.

14.  Security Considerations

   There are no additional security risks introduced by this design.

15.  References

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
              "Border Gateway Protocol (BGP) Persistent Route
              Oscillation Condition", RFC 3345, August 2002.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

Authors' Addresses

   Pedro Marques


   Rex Fernando
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134


   Enke Chen
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134


   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134


   Hannes Gredler
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089