Multi Protocol BGP, or MP-BGP, allows BGP to carry multiple network layer reachability information, NLRIs, for multiple protocols.
These multiple protocols can include IPv4, IPv6, Multiprotocol Label Switching (MPLS) and Layer 3 virtual private networks – L3VPNs.
These MP-BGP features are defined in RFC4760. RFC4760 defines the new address family identifier model (AFI), and new BGPv4 optional and non transitive attributes: Multi-protocol reachable NLRI, and Multi-protocol unreachable NLRI.
The multiprotocol reachable NLRI attribute describes IPv6 route information, and the multiple unreachable NLRI attributes withdraws the IPv6 route information from service. Both of these attributes are optional and non-transitive. This means if an older router does not understand or has compatibility for these attributes, the router can just ignore them.
With MP-BGP all the underlying IPv6 path-vector used in BGP still apply to MP-BGP. Port 179 is still used for IPv6 in the same way that it is used for IPv4.
When MP-BGP negotiates with a neighbour, it’s initial open message will exchange the peer capabilities with its neighbour. This exchange will include the address family identifier that describes supported protocols including the subsequent address family identifiers.