When an eBGP neighbor forwards routing information to another eBGP neighbor,
it changes the next hop to itself. When an iBGP neighbors exchange
information they do not, by default, change the next hop. This is where the
synchronization rule comes in.
An iBGP neighbor will not be able to use a route if it does not have a valid
route to the next hop in its IGP. Having synchronization turned on is often
unnecessary, so most people turn it off. You still have a problem, though:
the receiving iBGP neighbor still might not know how to reach the next hop
for any of the routes in its BGP table. To solve this, on your iBGP peers
use the next-hop-self command. Since the peers already know how to reach
each other, this solves your problem.
I hope that helps, and I hope I haven't mischaracterized the issue. I
haven't really thought through all of this in a while so I may have some
details wrong.
If you really want to understand this stuff, pick up a copy of Internet
Routing Architectures (2nd Ed.) by Sam Halabi.
Another book I really liked is short but sweet. It's BGP4: Interdomain
Routing in the Internet (or something close to that.) It's very short but
it's an excellent resource. Perhaps you should read that first and then
read Halabi.
Or you could also get a subscription to Certification Zone and read Howard's
papers on BGP, they're quite excellent.
HTH,
John
> I'm really confused about the how Next-hop attribute works for IBGP and
> EBGP. Can somebody please shed some light on this. Any tips or help
> would be greatly appreciated.
>
> Regards,
> Hunt
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