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Thread Index [isp-bgp] Route advertisement problem
For the past several years, we have been running BGP with 3 different upstream
ISPs--Qwest, UUNET, and Sprint. All three of these links are T1 circuits,
which terminate at the same router on our side. We have two /24s from Sprint,
and one /24 from UUNET. We are using AS prepending to load share incoming
traffic among the three circuits.
Well, recently, we decided that it was time for more bandwidth. For purposes
of redundancy and cost, we decided to run some high bandwidth wireless links
into another city, and backhaul connectivity from there. In the other city,
we will have a 4xT1 from AT&T.
Anyway, I'm now in the process of trying to figure out the routing for this
whole mess, and I've run into a bit of a dilemma. It would probably be
easiest to illustrate with an ugly ASCII art network diagram. This is *very*
simplified, and is not what our network looks like, but it should be enough to
show the problem I'm facing:
AT&T Qwest MCI Sprint
| \ | /
| Wireless link 1 Wireless link 2 Wireless link 3 \ | /
*-----------------*-----------------*------------------*---Dialup ntwk
A | | B| 1.1.1.128/25
| | |
* * *---Office ntwk
| | |
| | |
1.1.1.0/26 1.1.1.64/26 Servers
(Wireless customer networks)
In the diagram above, all the asterisks (*) are routers. The routers labeled
A and B are edge routers. B is at the NOC and A is where the AT&T circuits
terminate in the neighboring city. All interior routing is done with OSPF,
and obviously, eBGP will be at the edges. iBGP will be run between A and B.
Let's say AT&T gives me a new /24 of 1.1.1.0/24, which I subnet into two /26
networks (1.1.1.0/26, and 1.1.1.64/26) and one /25 (1.1.1.128/25). I allocate
the two /26 blocks to my wireless customer networks, and the remaining /25, i
assign to my dialup network.
Now because the AT&T circuit is a 4xT1, I want most of the incoming traffic to
travel over that circuit, even though it may not be the shortest path. So
I'll use AS prepending on the Qwest, MCI, and Sprint circuits, so that
1.1.1.0/24 has a longer path over those circuits. I think that all of this
should work without any trouble.
Now here's the dilemma: If Wireless link 1 goes down, things will be fine.
Edge router A will stop announcing 1.1.1.0/24 to AT&T because it will no
longer have a route to any 1.1.1.x networks. However, in the event that
Wireless link 2 goes down, Edge router A will continue to announce the
1.1.1.0/24 route to AT&T because it can still see the 1.1.1.0/26 network.
Even though it cannot see 1.1.1.64/26 or 1.1.1.128/25. So basically, half of
my network will be down if Wireless link 2 dies. Instead, what I would like
to see is traffic to 1.1.1.64/26 and 1.1.1.128/25 come in via MCI, Qwest or
Sprint, while still having traffic to 1.1.1.0/26 come in through AT&T. But
the only way I can think of for that to happen would be to announce networks
smaller than /24 to my upstreams (which obviously cannot and would not be
allowed).
Is there any way around this problem, or am I stuck with it given the network
layout? I've sort of toyed with the idea of somehow using conditional route
advertisement to have edge router B only start advertising 1.1.1.0/24 if it
loses a route to edge router A. However, that still doesn't solve all of the
problems.
Anyway, sorry for the length of this post. It's sort of a complicated problem
to describe. Thanks for any help or advice.
Craig
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