8. Use of BGP Capability Advertisement

A BGP speaker that uses Multiprotocol Extensions SHOULD use the Capability Advertisement procedures [BGP-CAP] to determine whether the speaker could use Multiprotocol Extensions with a particular peer.

The fields in the Capabilities Optional Parameter are set as follows. The Capability Code field is set to 1 (which indicates Multiprotocol Extensions capabilities). The Capability Length field is set to 4. The Capability Value field is defined as:

                  0       7      15      23      31
                  +-------+-------+-------+-------+
                  |      AFI      | Res.  | SAFI  |
                  +-------+-------+-------+-------+

The use and meaning of this field is as follow:

AFI - Address Family Identifier (16 bit), encoded the same way as in the Multiprotocol Extensions

Res. - Reserved (8 bit) field. SHOULD be set to 0 by the sender and ignored by the receiver.

Note that not setting the field value to 0 may create issues for a receiver not ignoring the field. In addition, this definition is problematic if it is ever attempted to redefine the field.

SAFI - Subsequent Address Family Identifier (8 bit), encoded the same way as in the Multiprotocol Extensions.

A speaker that supports multiple <AFI, SAFI> tuples includes them as multiple Capabilities in the Capabilities Optional Parameter.

To have a bi-directional exchange of routing information for a particular <AFI, SAFI> between a pair of BGP speakers, each such speaker MUST advertise to the other (via the Capability Advertisement mechanism) the capability to support that particular <AFI, SAFI> route.