Capabilities Advertisement with BGP-4
RFC 5492

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Network Working Group                                         J. Scudder
Request for Comments: 5492                              Juniper Networks
Obsoletes: 3392                                               R. Chandra
Category: Standards Track                                  Sonoa Systems
                                                           February 2009

                 Capabilities Advertisement with BGP-4

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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   than English.

Abstract

   This document defines an Optional Parameter, called Capabilities,
   that is expected to facilitate the introduction of new capabilities
   in the Border Gateway Protocol (BGP) by providing graceful capability
   advertisement without requiring that BGP peering be terminated.

   This document obsoletes RFC 3392.

Scudder & Chandra           Standards Track                     [Page 1]
RFC 5492               Capabilities Advertisement          February 2009

1.  Introduction

   The base BGP-4 specification [RFC4271] requires that when a BGP
   speaker receives an OPEN message with one or more unrecognized
   Optional Parameters, the speaker must terminate the BGP peering.
   This complicates the introduction of new capabilities in BGP.

   This specification defines an Optional Parameter and processing rules
   that allow BGP speakers to communicate capabilities in an OPEN
   message.  A pair of BGP speakers that supports this specification can
   establish the peering even when presented with unrecognized
   capabilities, so long as all capabilities required to support the
   peering are supported.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Overview of Operations

   When a BGP speaker [RFC4271] that supports capabilities advertisement
   sends an OPEN message to its BGP peer, the message MAY include an
   Optional Parameter, called Capabilities.  The parameter lists the
   capabilities supported by the speaker.

   A BGP speaker determines the capabilities supported by its peer by
   examining the list of capabilities present in the Capabilities
   Optional Parameter carried by the OPEN message that the speaker
   receives from the peer.

   A BGP speaker that supports a particular capability may use this
   capability with its peer after the speaker determines (as described
   above) that the peer supports this capability.  Simply put, a given
   capability can be used on a peering if that capability has been
   advertised by both peers.  If either peer has not advertised it, the
   capability cannot be used.

   A BGP speaker determines that its peer doesn't support capabilities
   advertisement if, in response to an OPEN message that carries the
   Capabilities Optional Parameter, the speaker receives a NOTIFICATION
   message with the Error Subcode set to Unsupported Optional Parameter.
   (This is a consequence of the base BGP-4 specification [RFC4271] and
   not a new requirement.)  In this case, the speaker SHOULD attempt to
   re-establish a BGP connection with the peer without sending to the
   peer the Capabilities Optional Parameter.

Scudder & Chandra           Standards Track                     [Page 2]
RFC 5492               Capabilities Advertisement          February 2009

   If a BGP speaker that supports a certain capability determines that