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    Understanding Route Selection When Route-Target Filtering Is Enabled

    When route-target filtering is enabled for a peer, BGP applies outbound filters to initially prevent the speaker from advertising any VPN routes to the peer.

    If the BGP speaker subsequently receives a Default-MEM-NLRI route from a peer, BGP applies outbound filters for the peer to prevent route-target filtering from suppressing any VPN routes sent to the peer.

    BGP follows the standard route selection process to find the route-target filtering best path for RT-MEM-NLRI routes received from other autonomous systems. The selection is based on the AS path and other MP-NLRI path-attributes attached to the route.

    The route-target membership information, which includes the route target and the originator AS number, enables BGP speakers to use the standard path selection rules to remove duplicate, less-preferred paths from the total set of paths to route-target membership peers.

    For RT-MEM-NLRI routes that originated within the local AS and are received from an IBGP peer, BGP considers the route-target filtering best path to be the set of all available IBGP paths for the RT-MEM-NLRI prefix. BGP then sets outbound route filters so that VPN routes that match the route target are sent to all IBGP peers that advertised the RT-MEM-NLRI route. This behavior does not affect how the BGP speaker in turn advertises the RT-MEM-NLRI routes.

    When BGP selects a RT-MEM-NLRI route from a peer as the best path for the RT-MEM-NLRI prefix, BGP modifies the outbound filters to enable the speaker to advertise to that peer all VPN routes that correspond to that route target. These filters affect the subsequent calculation of the peer's Adj-RIBs-Out entries.

    EBGP confederation peers are treated as IBGP peers when the BGP speaker is selecting the route-target filtering best path. When the BGP speaker advertises routes, then the EBGP confederation peers are treated normally, as EBGP peers.

    You can control the maximum number of received EBGP best paths that are considered for path selection. The external-paths command limits external route target membership, thus controlling the number of EBGP peers that receive the route target VPN routes referenced by the RT-MEM-NLRI route. BGP ignores routes received from the peer after the limit specified with the external paths command is reached.

    Published: 2014-08-18