Multi-protocol BGP attributes

As discussed in the BGP chapter overview the uses of BGP have increased well beyond Internet IPv4 routing because of its support for multi-protocol extensions, or more simply MP-BGP. MP-BGP allows BGP peers to exchange routes for NLRI other than IPv4 prefixes - for example IPv6 prefixes, Layer 3 VPN routes, Layer 2 VPN routes, FlowSpec rules, and so on. A BGP router that supports MP-BGP indicates the types of routes it wants to exchange with a peer by including the corresponding AFI (Address Family Identifier) and SAFI (Subsequent Address Family Identifier) values in the MP-BGP capability of its OPEN message. The two peers forming a session do not need to indicate support for the same address families. As long as there is one AFI/SAFI in common the session establishes and routes associated with all the common AFI/SAFI can be exchanged between the peers.

The list of AFI/SAFI advertised in the MP-BGP capability is controlled entirely by the family commands. The AFI/SAFI supported by the SR OS and the method of configuring the AFI/SAFI support is summarized in Table: Multi-protocol BGP support in SR OS.

To advertise reachable routes of a particular AFI/SAFI a BGP router includes a single MP_REACH_NLRI attribute in the UPDATE message. The MP_REACH_NLRI attribute encodes the AFI, the SAFI, the BGP next-hop and all the reachable NLRI. To withdraw routes of a particular AFI/SAFI a BGP router includes a single MP_UNREACH_NLRI attribute in the UPDATE message. The MP_UNREACH_NLRI attribute encodes the AFI, the SAFI and all the withdrawn NLRI. While it is valid to advertise and withdraw IPv4 unicast routes using the MP_REACH_NLRI and MP_UNREACH_NLRI attributes, SR OS always uses the IPv4 fields of the UPDATE message to convey reachable and unreachable IPv4 unicast routes.