Understanding Route-Target Filtering for MBGP VPNs Overview

In typical BGP configurations, you can use cooperative route filtering to reduce the amount of processing required for inbound BGP updates and the amount of BGP control traffic generated by BGP updates. Cooperative route filtering works by having the remote peer install a BGP speaker’s inbound route filter as its own outbound route filter. This filtering causes the remote peer to advertise only those routes that the local peer can accept.

For BGP/MPLS VPNs, route-target filtering is a better approach. Route-target filtering controls the distribution of BGP routes based on the VPNS (indicated by the route-target extended communities) to which peer routers belong. PE routers use the MP_REACH_NLRI and MP_UNREACH_NLRI attributes in BGP updates to exchange information about each router’s route-target membership.

The PE router subsequently advertises VPN NLRI—the routing information carried in MP-BGP update messages—only to peers that are members of a route target that is associated with the VPN route. The VPN routes flow in the opposite direction to the route-target membership information.

Route-target filtering works across multiple ASs and with asymmetric VPN topologies, such as a hub-and-spoke. Route-target filtering can reduce the size of the BGP routing table in PE routers, as well as the amount of VPN NLRI exchange traffic between routes in the VPN. Route-target filtering also reduces router memory requirements by reducing the amount of routing information stored and propagated. For example, route reflectors scale according to the total number of VPN routes present in their network. With route-target filtering, you can reduce the scaling requirements of the reflectors by restricting the number of VPN routes they must process to only those VPN routes actually used by the route reflector clients.

Applications such as BGP/MPLS VPNs, VPLS L2VPNs, and VPWS L2VPNs all use route targets as part of their route reachability information, and can therefore employ route-target filtering and potentially accrue the benefits of reduced traffic and smaller routing tables.

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