Re: Next HOP on IBGP

As I understand your description, this is normal behavior. Think about it
from these terms:

iBGP is run internal to your network. Everyone in your network can reach
each other, because you have an IGP running. If you have reachability
issues internal to your network (like in the lab), then you may need to
change the next hop to self in order to use a next hop ip address that the
other routers know.

This is a fun one to tshoot because your peering is just fine, but because
the router does not know how to get to the next hop, it can not use the
route. Ping and show ip bgp commands can help ... also helps to look over
your network diagram and see who is connected to who ...

How do iBGP peers get to external routes, ... they use a iBGP router that
also has the eBGP peering sessions. The router w/ the eBGP peering session
is advertising the routes it learned from its eBGP peers. iBGP routers
cannot be expected to know of external routes and how to get there, so the
eBGP router changes the next hop to itself, and then advertises the routes
internally.

The eBGP router is saying to it's iBGP peers - "to get to network xyz, come
to me". Make sense? In your case, R2 is telling R3 to come to R2 in order
to get to the external routes that are advertised by R1. I hope I have not
been too confusing.

Here is a case study that might help:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml

If you have some labs, this will greatly help and you can use the above link
to practice some scenarios as well. IMO, BGP theory is nearly impossible to
learn without doing it ... but then again, I am "a bear of little brain" to
quote Winnie the pooh.

HTH,

Andrew Lee Lissitz

On Tue, Aug 18, 2009 at 5:46 AM, Divin Mathew John <divinjohn_at_gmail.com>wrote:

> Could you paste the config?
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Andrew Lee Lissitz
all.from.nj_at_gmail.com
Blogs and organic groups at http://www.ccie.net