Controlling advertisements, such as Border Gateway Protocol (“BGP”) updates, of multiple paths for a given address prefix

摘要

BGP can advertise multiple routes for same prefix via BGP add path (RFC 7911). BGP attempts to pack prefixes with same path attributes into the same BGP update message. Protocol nexthop is one of the path attributes. Since these BGP add paths routes usually have different protocol nexthops, different routes for a single prefix could end up being spread out when being advertised. That may, in turn, result in additional calls to download routes to FIB, advertisement to peers and multiple runs of multipath calculation for the same prefix when multipath is configured. To help avoid this situation, when BGP advertises add-path routes, BGP can send the multiple paths for the same prefix in the adjacent update messages. BGP can use extended Network Layer Reachability Information (NLRI) field to carry nexthop along with its associated prefix in BGP update message to send plain IPv4 unicast routes. In addition, BGP reachability information can be encoded in MP_REACH_NLRI attribute with which protocol nexthop information is carried along with its own prefix in the extended “NLRI” field. A BGP speaker can use some data structure to link these routes for the same prefix and put multiple routes for the same prefix in adjacent or even same BGP update messages.