BGP Route Selection Criteria

BGP route selection criteria (on Cisco router) :

1. Exclude route with inaccessible next-hop
2. Prefer highest weight (local to router)
3. Prefer highest local preference (globa within AS)
4. Prefer routes that router originated
5. Prefer shortest AS paths (only length is compared)
6. Prefer lowest origin code (IGP
7. Prefer lowest MED
8. Prefer external (EBGP) path over internal (IBGP)
9. For IBGP paths, prefer path through closest IGP neighbor
10. For EBGP paths, prefer oldest (most stable) path
11. Prefer paths from router with the lowest BGP router-id


The route selection criteria means, for example, when one prefix is learning from three upstream, A with 2 as paths, B with 3 paths, and C with 5, with weight, and local preference is the sama, it will choose A for next hop. It is usually weight, and local preference is same until we change it manipulate the routing. When we want to change preference of some prefixes, you can use weight, if the gateway just one, or when many router gateway in your AS, you need use local preference, because local preference affect in local AS, while weight affect in local router only.