Internal BGP (Border Gateway Protocol) explained

Hello Vivek

Remember that in order to correctly configure iBGP within an AS, you must first ensure that you have successful routing between all routers within that AS using an IGP or using static routing. Make sure that the loopbacks on all of your routers are reachable from all other loopbacks in the network. Also, keep in mind that iBGP requires that you create BGP peers not only between directly connected routers but between ALL routers. So in your topology, each iBGP router should have three peerings.

One of the things that I found confusing when learning about BGP is the fact that you need an IGP to make BGP work. But why? Isn’t BGP a routing protocol itself? We must remember the role of BGP. BGP is not responsible for the routing found within an AS. BGP is responsible for advertising the prefixes within the AS to external ASes. iBGP shares them within the AS, and eBGP takes those collected prefixes and shares them with other ASes.

So iBGP is not responsible for ensuring that you have connectivity between your loopbacks. You must ensure that this is achieved using an IGP or static routing before you employ BGP.

I hope this has been helpful!