Network Working Group R. Chandra
Request for Comments: 2842 Redback Networks Inc.
Category: Standards Track J. Scudder
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.
Copyright (C) The Internet Society (2000). All Rights Reserved.
Currently BGP-4 [BGP-4] requires that when a BGP speaker receives an
OPEN message with one or more unrecognized Optional Parameters, the
speaker must terminate BGP peering. This complicates introduction of
new capabilities in BGP.
This document defines new Optional Parameter, called Capabilities,
that is expected to facilitate introduction of new capabilities in
BGP by providing graceful capability advertisement without requiring
that BGP peering be terminated.
1. Overview of Operations
When a BGP speaker 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.
Chandra & Scudder Standards Track [Page 1]RFC 2842 Capabilities Advertisement with BGP-4 May 2000
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.
In this case the speaker should attempt to re-establish a BGP
connection with the peer without sending to the peer the Capabilities
If a BGP speaker that supports a certain capability determines that
its peer doesn't support this capability, the speaker may send a
NOTIFICATION message to the peer, and terminate peering. The Error
Subcode in the message is set to Unsupported Capability. The message
should contain the capability (capabilities) that causes the speaker
to send the message. The decision to send the message and terminate
peering is local to the speaker. Such peering should not be re-
2. Capabilities Optional Parameter (Parameter Type 2):
This is an Optional Parameter that is used by a BGP speaker to convey
to its BGP peer the list of capabilities supported by the speaker.
The parameter contains one or more triples <Capability Code,
Capability Length, Capability Value>, where each triple is encoded as
| Capability Code (1 octet) |
| Capability Length (1 octet) |
| Capability Value (variable) |
The use and meaning of these fields are as follows:
Capability Code is a one octet field that unambiguously
identifies individual capabilities.
Capability Length is a one octet field that contains the length
of the Capability Value field in octets.
Chandra & Scudder Standards Track [Page 2]RFC 2842 Capabilities Advertisement with BGP-4 May 2000
Capability Value is a variable length field that is interpreted
according to the value of the Capability Code field.
A particular capability, as identified by its Capability Code, may
occur more than once within the Optional Parameter.
3. Extensions to Error Handling
This document defines new Error Subcode - Unsupported Capability.
The value of this Subcode is 7. The Data field in the NOTIFICATION