From: John Hever (john.hever@xxxxxxxxxxxxxxxxxxxxxxxxxxxx)
Date: Fri Aug 24 2001 - 15:07:47 GMT-3
Hi Brent,
OK here is my understanding:
The neighbor x.x.x.x update-source loopback x command is usually used
between IBGP peers when there is more than one physical path between them.
This ensures that the loopback interface address specified in the
'update-source' command is used as the source address in IBGP packets, this
ensures that the IBGP session is not reset (as it would be if we had not
specified an update-source address, because the IBGP packets would have used
the physical interface's IP address) if one of the physical interfaces
fails.
EBGP peerings are usually via a single serial link so the situation does not
arise.
A clearer, concise example is available in the Cisco BGP-4 Command and
Configuration Handbook, on page 235.
Other than that the configs you have posted should work, apart from the nei
10.0.0.2 ebgp-m 2 statement which I assume is a typo, and should say nei
10.0.0.1 ebgp-m 2.
HTH
John
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Brent D. Stewart
Sent: 24 August 2001 18:19
To: ccielab@groupstudy.com
Subject: BGP - why does this work?
I've got two routers that are directly attached with an HDLC circuit in my
lab. Each run BGP, one uses the serial address of the other in it's
neighbor statement; the other peers to a loopback interface.
R1--------------------------R2
s0 192.168.0.1/24 lo0 10.0.0.1/32
The R1 configuration looks like
router bgp 1
nei 10.0.0.1 remote-as 2
nei 10.0.0.2 ebgp-m 2
The R2 configuration looks like
router bgp 2
nei 192.168.0.1 remote-as 1
RIP is running between them as well, so R1 knows how to get to R2's
loopback.
Why does this work? I thought I needed to add to R2's configuration "nei
192.168.0.1 update-s lo0". I thought that R1's BGP process would check the
source address before peering (btw - adding the statement doesn't break it).
Obviously I thought wrong, because it works. R1 is running 12.0(10) and R2
is running 12.0(3c), but I've duplicated this with a couple of intervening
versions. Is this a v12 protect-them-from-themselves thing, or did I just
never understand this correctly?
Thanks everyone.
Regards,
Brent D. Stewart, CCSI
Global Knowledge
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