BGP Path Selection

BGP Path Selection

BGP can receive multiple advertisements for the same route from multiple sources. BGP selects only one path as the best path. After the path is selected, BGP puts it in the IP routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order listed, to select a path for a destination:

• Ignore a route if the next hop is not known.

• Ignore IBGP routes that are not synchronized.

• Prefer the route with the largest weight.

• Prefer the route with the largest local preference.

• Prefer the route that was locally originated.

• Prefer the route with the shortest AS path. If you're using bgp bestpath as-path ignore, skip this step. When you use the as-set option for aggregated routes, it counts as 1 regardless of the number of AS entries in the set. Confederation sub-AS numbers are not used to determine the AS-path length.

• Prefer the route with the lowest origin (IGP < EGP < Incomplete).

• Prefer the route with the lowest MED. This comparison is only between routes advertised by the same external AS.

  If you're using bgp always-compare-med , compare MEDs for all paths. If used, this command needs to be configured on every BGP router in the AS.

  If you're using bgp bestpath med- confed , the MEDs are compared only for routes that have an AS confederation sequence in their AS-path attribute.

  If a prefix is received with no MED value, it is assigned a MED value of 0. If you're using bgp bestpath med missing-as-worst, a prefix with a missing MED value is assigned a MED value of 4,294,967,294.

• Prefer EBGP routes to IBGP routes.

• Prefer the route with the nearest IGP neighbor.

• Prefer the oldest route.

• Prefer the path received from the router with the lowest router ID.