From: Joseph Brunner (joe@affirmedsystems.com)
Date: Mon Aug 11 2008 - 09:08:03 ART
R6 See's the routes from both R3 and R5 but it sees all next hop-addresses
as R5.
Why is this?
>R6 is choosing R5 as the *best* route for the routes originally learned by
>R3/R5 from R1, BB1
You are seeing the correct behavior.
All the paths should have two entries
Such as
Show ip bgp
1.1.1.0, 3.3.3.3
>1.1.1.0, 5.5.5.5
But the ">" is on the routes to R5, yes?
When R3 does a recursive look up for
the next-hop-address it sees it via R5 and hence this is why it's
advertising the bgp routes to R6 with the next-hop address of R5?
>not exactly. EBGP peers simply REset the next hop to the update source by
default, IMHO. However since you mentioned this is on a broadcast segment,
its possible.
Can you try to make R6 only directly adjacent to R3 (say via a p2p link in
Dynamips), remove the R6 -> R5 peering and see what the next-hop's look
like...
Deep down I remember something like this YEARS back on a /24 frame-relay
cloud!
-Joe
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
CCIE3000
Sent: Monday, August 11, 2008 5:29 AM
To: Cisco certification
Subject: Silly BGP next hop question
Hi Guys,
I've got a silly question concerning next-hop with bgp
R1 Peers with BB1
R1 is the route-reflector for R3 and R5
They are peered via their loopbacks.
They see the bgp routes with the next-hop-address of BB1.
R3 and R5 then have an EBGP peering with R6.
R6 See's the routes from both R3 and R5 but it sees all next hop-addresses
as R5.
Why is this? okay, it's an EBGP peering so the next-hop value should be the
ebgp peers.
R3 must go via R5 to get to the BGP routes learnt from the BB router.
R3, R5 and R6 are all on the same subnet.
Is it because in it's BGP table R3 sees the bgp routes with the next-hop of
the BB router (because it's learnt from the Route Reflector which is ibgp
peering so the next-hop won't change). When R3 does a recursive look up for
the next-hop-address it sees it via R5 and hence this is why it's
advertising the bgp routes to R6 with the next-hop address of R5?
I hope the above is clear enough and not too confusing.
Thanks Si.
Blogs and organic groups at http://www.ccie.net
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