Understanding Route Targets

A route-target extended community, or route target, is a type of BGP extended community that you use to define VPN membership. The route target appears in a field in the update messages associated with VPN-IPv4.

You create route-target import lists and route-target export lists for each VRF. The route targets that you place in a route target export list are attached to every route advertised to other PE routers. When a PE router receives a route from another PE router, it compares the route targets attached to each route against the route-target import list defined for each of its VRFs. If any route target attached to a route matches the import list for a VRF, then the route is imported to that VRF. If no route target matches the import list, then the route is rejected for that VRF.

Depending on your network configuration, the import and export lists may be identical. Typically, you do the following:

For more complicated scenarios—for example, hub-and-spoke VPNs—the route-target import list and the route-target export list might not be identical.

A route-target import list is applied before any inbound routing policy (route map) is applied. If an inbound route map contains a set extcommunity clause, the clause replaces all extended communities in the received route. BGP applies the default route-target export list associated with the VRF if the route does not have any route-target extended-community attributes after the inbound policy has been applied. On the other hand, the default export list is not applied if either a valid route-target export list is received or the inbound route map sets one or more route targets.

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