CCNP ROUTE Exam Tutorial:
Using The BGP Command Update-Source
By Chris Bryant, CCIE #12933
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When you start preparing for your CCNP ROUTE exam, you're introduced to Border Gateway Protocol (BGP).
BGP is unlike any protocol you learned during your CCNA studies, and even the similarities are a little bit different!
A note for those of you just starting on BGP or perhaps struggling a bit with it - I ran into the exact same situation when I began studying for my CCNP. If you look at BGP as a whole, it will overwhelm you - and if you just take one little concept at a time, your knowledge will build until you've mastered the subject.
So hang in there - it gets better.
Now back to work on this command...
BGP forms neighbor relationships, much like EIGRP and OSPF do. The interesting thing with BGP is that potential neighbors, or "peers", do not need to be directly connected and can use their loopback interfaces to form the peer relationships.
It may well be to your advantage to use loopbacks to form peer relationships rather than the actual interface facing the potential neighbor. This can be done because BGP uses static neighbor statements rather than any kind of dynamic neighbor discovery process.
Consider a router that has two paths to a BGP speaker. The interfaces are numbered like this:
Serial0, 220.127.116.11 /24
Serial2, 18.104.22.168 /24
loopback0, 22.214.171.124 /32
loopback0, 126.96.36.199 /32
We could configure Router1 like this:
router bgp 200
neighbor 188.8.131.52 remote-as 200
In this case, BGP would automatically use 184.108.40.206 as the source for the TCP connection that has to be set up with the neighbor before updates can be exchanged; this address is known as the best local address.
However, if the remote peer's serial0 interface is shut down or goes down for another reason, the peer relationship would be lost even though Router2 is still up.
Instead of using one of the physical interfaces, we can use the loopbacks on each router to establish the TCP-based peer connection. The configurations would look like this:
router bgp 200
neighbor 220.127.116.11 remote-as 200
neighbor 18.104.22.168 update-source loopback0
router bgp 200
neighbor 22.214.171.124 remote-as 200
neighbor 126.96.36.199 update-source loopback0
In this case, losing one of the physical connections does not necessarily mean the BGP peering is lost; as long as the routers have a valid path to each other's loopback addresses, the BGP peer relationship will stay in place. And better yet, we avoid the dreaded “single point of failure”!
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