Tnx a lot Scott. I'm still not there. The more I think about it, the more I get confused. I only have more questions. You remark about the AS# cleared up some things though. Obviously the AS-path attribute is the only means by which BGP is able to tell where a routing update contains a loop, that is going thru different AS systems.
One thing you write is that "IBGP neighbors will not propagate routes to each other as a matter of loop protection". Further on in your example you state that "R1 will forward it's routes to R2. R2 will forward it's routes to R3". So I am confused between the difference between "propagate" (not done) and "forwarding" (done). If a router, running EBGP on one interface and IBGP on another, has learned an external route (EBGP out of its AS), it will forward this route to all IBGP peers, doesn't it? And since IBGP peers are either fully meshed or clustered, each IBGP route will learn the EBGP external route directly, not via another IBGP peers, but directly from the router running EBGP and IBGP, don't they? Exactly what internal routers will IBGP peers "forward" to each other except EGBP routes? Only IGP routes? In this case, IBGP learned routes will not be propagated unless they are learned by IGP as well, aren't they? So if each router already has learned the route(s) by IGP, why bother with learning them from IBGP anyway? Sorry about hasseling you like this. I did reread the BSCN book, but it didn't make thing clearer, because it mostly state "what is the case", rather then "why it is the case". Joep, (CCNA, CCDA) BTW: The notion of BGP split horizon, I think I got this one from the BSCN book. BGP split horizon implies that those routes that are learned via IBGP are not propagated to other IBGP peers, meaning you will need to have a full-mesh IBGP. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=35722&t=35679 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
