"Internal BGP is a mechanism to provide more information to your
internal routers. Most of last week’s installment on
Understanding BGP focused on a stub configuration, where a single
router served all the BGP sessions for an autonomous system (AS).
This time we’ll delve into the practical use of BGP: iBGP and
what it takes to accomplish multihoming.
"If you were to add a second BGP router and connect it to
another peer, your network wouldn’t gain much until the IGP
knew what to do. There are a few options here, and one is a grave
mistake. You cannot simply redistribute all of the Internet routes
into your IGP and hope for the best. It’s really fun to do,
actually, because the OSPF process normally takes down the router.
Also, you need to get the routes learned from one border router to
another, but that information will be lost unless both border
routers are speaking BGP.
"The solution is to set up an internal BGP peering between all
of your border routers. The conventional wisdom is that your
network will consist of a core (or transit, or backbone, or
whatever you’d like to call it) network where this iBGP runs,
and a default route will be injected into the widely used IGP (OSPF
or other). As long as the IGP gets packets into the backbone, the
routers there will be able to choose the best exit strategy."