From: ccie2be (ccie2be@nyc.rr.com)
Date: Sat Aug 07 2004 - 23:00:51 GMT-3
Larry,
Here's the key concept you must completely understand.
For 2 bgp routers to peer, a symetric tcp session has to be established
between them. By default, a bgp router will use as the source address of
packets it sends to a bgp peer the ip address of the interface used to send
out the packets. The destination address it uses is, of course, the address
used in the nei x.x.x.x remote-as statement.
Therefore, let's say that the source address is S and the destination
address is D for rtrX.
If rtr X is peering with rtr Y, then rtr Y, has to use the same source and
destination addresses except that now those addresses are swapped.
Therefore, if the source address of rtr X is not the destination address rtr
Y uses (and vice versa), the peering won't work.
Now, let's say you want the loopback address to be the source address used
in packets sent to a bgp peer. That's fine as long as the peer specifies
that source address in it's neighbor statement.
However, if these peers are in different AS's, then you're not done just
yet. This is because Ebgp expects peers to use the directly connected
interface. If you're not using the directly connected interface, then you
have to also add the neighbor <ip ad> ebgp multihop # command to both bgp
peers.
If, on the other hand, both bgp routers are in the same AS, you don't need
this command. But, the question remains, why would you want to use a
non-default interface like the loopback interface for your internal bgp
peering sessions?
The reason is to increase bgp stabililty. If there are multiple paths known
via the IGP, you don't really care which path is used for your I-bgp
sessions. Either path from BGP's point of view is fine. So, if you use the
loopaback interface as the endpoints of your ibgp session, if one path
becomes unavailable, the IGP will just use the other path. And, your iBGP
peering will remain intact.
HTH, Tim
----- Original Message -----
From: "Larry Metzger" <larrymetzger@sbcglobal.net>
To: "Group Study" <ccielab@groupstudy.com>
Sent: Saturday, August 07, 2004 8:10 PM
Subject: BGP Update Source
> I am working on BGP configurations. I have Router1 (as100) and Router2
> (as200). I'm using the loopback on Router1 for the neighbor statement.
> The recommendation is to add "neigh R2 update-source loop0" I have
> configured these routers with and without the update source and
> everything works fine either way (sh ip bgp, sh ip bgp nei, sh tcp). Is
> it necessary? What problems will it cause if you don't change the
> update-source?
>
> Larry
>
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