BGP Capabilities Negotiation

While the BGP Capabilities Negotiation specification is still being developed by the IETF IDR (Inter-Domain Routing) Working Group^[5], we'll touch on its usefulness here in order to foster understanding for future discussions of new mechanisms that utilize the facilities it provides in order to introduce new features to BGP.

The Capabilities Negotiation with BGP-4 specification is currently in draft form and is still a "work in progress." The purpose of this specification is to introduce a new optional parameter to BGP-4 called Capabilities. It is expected to facilitate the introduction of new features in BGP by providing graceful capability negotiation without requiring that the BGP peering be terminated.

If a BGP speaker supports capabilities negotiation, when it sends an OPEN message to a BGP peer, the message may include an Optional Capabilities parameter. A BGP speaker examines the information contained in the Capabilities parameter of an OPEN message in order to determine which capabilities the peer supports. If a BGP speaker determines that a peer supports a given capability, the speaker can use the capability with the peer.

A BGP speaker determines that a peer doesn't support capabilities negotiation if, in response to an OPEN message that carries the new Optional parameter, the speaker receives a NOTIFICATION message that contains an Error Subcode set to "Unsupported Optional Parameter." If this occurs, the BGP speaker should attempt to reestablish the connection without sending the Capabilities Optional parameter to the peer.

BGP Capabilities is of Parameter Type 24 and contains one or more triples <Capability Code, Capability Length, Capability Value>, where each triple is encoded as illustrated in Figure 5-15.

The uses and meanings of the three fields are as follows:

• Capability Code— A 1-byte field that unambiguously identifies individual capabilities.

• Capability Length— A 1-byte field that contains the length of the Capability Value Field in bytes.

• Capability Value— A variable-length field that is interpreted according to the value of the Capability Code field.

Capability Code 0 is currently reserved. Capability codes 128 through 255 are also currently reserved for vendor-specific applications.

Cisco IOS already uses BGP Capabilities Negotiation to introduce two new features to BGP—BGP Route Refresh and Outbound Request Filter (ORF), both of which have already been implemented and will be discussed in more detail in later chapters.

BGP Capabilities provides a great deal of potential when attempting to introduce new features to BGP. It should be noted, however, that many such features might very well remain vendor-proprietary.