BGP Summary

BGP have iBGP for internal within same AS and eBGP for external for peering with different AS.
iBGP connectivity must full mesh configuration. If not, can used BGP Confederation (Sub AS in AS) and route reflector.


Inside iBGP must have IGP. Before going check iBGP, make sure the IGP working fine. For iBGP can used physical interface or loopback for peering. If using loopback need another command 'neighbor xxx update-source loopback z' and 'neighbor xxx next-hop-self' for routing pingable to eBGP.


eBGP must connect directly for peering. For eBGP need command 'neighbor xxx ebgp-multihop' if using loopback.


BGP Confederation need additional command 'bgp confederation identifier xx' which is xx is the real AS-number and command 'bgp confederation peers zzz' which is zzz is another sub AS number the peering directly.


In the route reflector, all client only peer to route reflector. In the route reflector, add command 'neighbor xxx route-reflector-client' which xxx is the client router.


Here route selection priority in Cisco:
1. Highest weight
2. Highest local preference
3. Route originated by the local router
4. Shortest AS-Path
5. Lowest origin code (IGP < EGP)
6. Lowest MED from other AS
7. eBGP over iBGP
8. The path trough the closest IGP neighbor
9. Oldest route for eBGP
10. Path with the lowest neighbour BGP Router ID
11. Path with the lowest neighbour IP