Network Working Group J. Scudder
Request for Comments: 5492 Juniper Networks
Obsoletes: 3392 R. Chandra
Category: Standards Track Sonoa Systems
Capabilities Advertisement with BGP-4
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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
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