Let's say AS100 is a multihomed AS, connected to AS200 and AS300. It has two eBGP border routers - B1 is connected to AS200, B2 to AS300. The IGP inside AS100 is OSPF.
If B1 knows about AS200, and B2 knows about AS300, and AS100 has IGP protocol running in it, why would anyone need internal BGP (iBGP)?
I thought that's the point of OSPF - it tells every router inside AS about the reachable networks and the right paths, so two hosts from any pair of networks can communicate. So if B1 knows about AS200, I thought that IGP just steps in and tell everyone else in AS100, including B2, about networks from AS100.
But for some reason, iBGP is still needed between B1 and B2 to make thigs fully work. Is it because B2 needs to know exactly the same thing as B1 and vice versa and IGP isn't telling B2 all that B1 knows (not sure why)?
I'd be grateful if you could explain it in the most straighforward way.