August 20, 2012 Leave a comment
1. Next_Hop: The next hop of the BGP route has to be in the routing table else if it is unreachable, the route is ignored.
2. Pre-bestpath Cost: If the pre-bestpath cost attribute is present, choose the route with the lowest cost value, if they are the same, the lowest community.
3. Weight: Cisco proprietary, local significant attribute where the largest is prefered.
4. Loc_Pref: If the weights are the same, choose the path with the highest local preference.
5. Local Originated: Routes that were locally originated with network statement, aggregated or redistributed.
6. AS_PATH: Next compare the as-path length and prefer the route with the shortest AS_PATH length.
7. ORIGIN: Choose the route with the lowest origin type if the AS_PATH lengths are the same. IGP<EGP<INCOMPLETE
8. MED: If the origin types are the same, choose the route with the lowest MED value. This will only be compared for routes from the same AS or if bgp always-compare-med is enabled, for all routes.
9. EBGP/iBGP: Prefer EBGP routes over IBGP routes if the routes have the same MED value.
10. IGP: At this point if there are still multiple routes prefer the route with the shortest route to the NEXT_HOP. The IGP will have already determined the shortest path to the next-hop.
11. Cost: If the cost attribute is present not configured to be ignored, choose the lowest cost.
12. Multipath: If multipath is enabled, multiple paths that match up to this point will be installed.
13. Oldest: If multiple external routes remain, choose the oldest one, thus avoids propagating a flapping route. To overwrite this, this step can be ignored with bestpath compare router-id.
14. Router_ID: If multiple routes still exist, the BGP ROUTER_ID will be a tiebreaker. Choose the route advertised by the BGP peer with the lowest Router_ID. If RR present, the originator ID is used.
15. Cluster Length: Minimum RR cluster length is compared next.
16. Lowest Neighbor: Last, the path from the lowest neighbor address.