From: Huan Pham (huan.pham@valuenet.com.au)
Date: Sun Jan 27 2008 - 21:23:18 ARST
Hi David,
I do not quite understand the topology you have, which make it very hard to
understand the exact problems you are facing.
Using best guess, below are my comments, which may be relevant to your case:
1 - Service providers normally replace the private AS peer with its own
public AS. This is to allow the same private AS number to be used at
multiple sites. Otherwise, there's a reachability issue, as BGP won't accept
route that has its own AS number in the AS path. This leads to the "ugly
design" asymmetric routing problem you mentioned, and that makes BGP routes
preferred by default to IGP routes. The solutions to this problem are:
Option1: Using BGP backdoor commands for all the routes you would like to
prefer using backdoor, which will override the Admin Distance behavior, and
will prefer IGP routes (90 for IEGRP) over eBGP (although with lower AD of
20)
Router bgp nnnnn
network x.x.x.x mask x.x.x.x backdoor
network x.x.x.x mask x.x.x.x backdoor
Option2: Using route-map to tweak AD for the those back-door routes, to make
eBGP routes less preferred than EIGRP
These two solutions should be implemented in the AT&T managed CE routers,
which should be AT&T responsibility, and not yours (customers routers).
Option3: Convert your routers to run BGP, and peers with AT&T CE routers via
eBGP (instead iBGP, i.e. using the same private AS as the one used by AT&T
CE). That way you can avoid full-mess requirement, and many other problem
related to iBGP.
I would prefer the Option1 the most, and Option2 is least recommended.
I could give you better advice if you could share the network diagram
(removing any sensitive details)
Cheers,
Huan
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
David
Sent: Sunday, 27 January 2008 8:44 AM
To: ccielab@groupstudy.com
Subject: To IBGP or not to IBGP....
hi,
BGP question.
I have been fighting with a BGP issue for a while and my customer does not
want to make the changes I recommend. The main change is a prefix filter to
prevent asymmetric routing. In addition to that I have many others. For
one, I wanted to replace EIGRP between the two BGP routers with an IBGP
relationship; however, I am getting push back on making any changes to their
environment. Besides the fact that I have always created IBGP relationship
between routers in the same AS, I was having problems thinking about major
issues or design constraints created by the use of EIGRP.
Current Design Facts:
Facts)
1. We have two BGP routers, AS 64646 (notice 666 in there?) That peers to
two distinct AT&T routers: Basic Multi-home single provider.
2) They run EIGRP between the two (64646) routers on a directly connected
interface on each router
3) they run EIGRP between another interface on each router and two distinct
6500s.
4) They each form EBGP relationship with provider on an ATM interface. Very
ugly because provider strips private AS and advertise our own network back
to us with a public AS. This fact sets the scene for the asymmetric routing.
IE, IGP routes disappear and EBGP is the best path to get to local
resources,
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