A Look at BGP

While almost all of the neighbor options are just that -- optional -- you do have to specify the BGP AS of the remote router. BGP has no mechanism to dynamically discover neighbors. Remember, BGP speakers do not have to be in the same AS to become peers. To verify that the remote BGP speaker has become a peer, run show ip bgp neighbor.



R1#show ip bgp neighbor



BGP neighbor is 172.12.123.3, remote AS 200, external link



BGP version 4, remote router ID 0.0.0.0



BGP state = Active



Last read 00:01:39, hold time is 180, keepalive interval is 60 seconds



Received 0 messages, 0 notifications, 0 in queue



Sent 0 messages, 0 notifications, 0 in queue



Route refresh request: received 0, sent 0



Default minimum time between advertisement runs is 30 seconds



The output here can be a little misleading the first time you read it. The first highlighted line shows 172.12.123.3 is a BGP neighbor, is located in AS 200, and is an external link, indicating that the neighbor is in another AS entirely. The second highlighted line shows the BGP state as Active. This sounds great, but it actually means that a BGP peer connection does not yet exist with the prospective neighbor.



So even though the show ip bgp neighbor output indicated that this is an Active neighbor relationship, that's not as good as it sounds. Of course, the reason the peer relationship hasn't been established is that we haven't configured R3 yet!



R3(config)#router bgp 200

R3(config-router)#neighbor 172.12.123.1 remote-as 100



Verify the peer establishment with show ip bgp neighbor:



R3#show ip bgp neighbor



BGP neighbor is 172.12.123.1, remote AS 100, external link



BGP version 4, remote router ID 172.12.123.1



BGP state = Established, up for 00:01:18



Last read 00:00:17, hold time is 180, keepalive interval is 60 seconds



Local host: 172.12.123.3, Local port: 179 (BGP uses TCP Port 179)



Foreign host: 172.12.123.1, Foreign port: 11007

The peer relationship between R1 and R3 has been established.



Now that you know how the neighbor relationship itself is built, you need to start learning the many options of the neighbor command. You'll have to master these to become a CCNP and CCIE!



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