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AS Path Prepend Problem

I am attempting to learn BGP by simulating various tasks in a lab environment.

*Please note that everything including IP's, AS numbers, etc are made up. Yes, I am mocking up a little internet type lab to learn this stuff and I was trying to make everything seem as real as it can be.

Problem: In the lab I am seeing the prepending correctly on the routers that I directly peer with. However, on any other router in my lab the prepend information is not passing through. From my reading it appears as if this information should stay in tact and be sent to all routers through BGP.

Details: I have a single router that is multihoming to two different AS'es (7018 and 209). This router will advertise 3 IP blocks

12.3.2.0/24 12.3.1.0/24 65.200.5.0/27

I am using the following route map to set the preference on which links I would prefer the ip addresses to use.

access-list 1 permit 12.3.2.0 0.0.0.255 access-list 2 permit 12.3.1.0 0.0.0.255 access-list 2 permit 65.200.5.0 0.0.0.31

route-map PrependToATT permit 10 match ip address 2 set as-path prepend 1000 1000 1000 ! route-map PrependToATT permit 20 match ip address 1 ! route-map PrependToQwest permit 10 match ip address 1 set as-path prepend 1000 1000 1000 ! route-map PrependToQwest permit 20 match ip address 2 !

On the routers that AS1000 directly peers with I see the path prepending properly. However, any other router (IBGP or EBGP) this information seems to be lost.

I have tried using soft reconfiguration to force the changes out but no luck.

What am I missing?

Thanks Amy.

Reply to
amyl
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If these routers receive the route from multiple peers, they'll only pass on the one with the shortest AS path. So if there's an advertisement without the prepending, it will be passed on in favor of the one with prepending.

Reply to
Barry Margolin

Let me see if I understand what you are saying:

Assume Router A is where I am doing the prepending. Sorry for the ASCII art.

A--B--E--| (A,B, and E = AS209) |--C--D---| (C,D = AS7018)

In Words: A Peers with B,C B Peers with E C Peers With D D Peers with E

Because the routes are advertised to B with no prepending they go through E to D. This would explain why on "D" I do not see the routes with prepending as it prefers the shortest AS path being advertised in from E.

If this is the case how do you advertise preferred paths using AS path prepending to prevent what is happening above. Or did I completely misunderstand what is occurring?

Thanks Amy.

Reply to
Amy L.

Right.

Prepending is used to make a path *not* be preferred. So if you want a path to be preferred, why are you prepending in the first place?

In other words, what are you attempting to accomplish?

Reply to
Barry Margolin

What I was trying to accomplish was making the routes

12.3.2.0/24 - Prefered via AS 7018 12.3.1.0/24 - Prefered via AS 209 65.200.5.0/27 - Prefered via AS 209.

So for example if I was in a different AS like 701 traffic to 12.3.2.0 would flow through 7018 and the other routes 12.3.1.0 and 65.200.5.0/27 from AS

701 would want to flow through AS 209. With the assumption that in my lap AS 701 peers with both 7018 and 209.

I figured that if I prepended to the AS path for the routes down the opposite peer where I wanted the routes to flow it would accomplish what I wanted to do in the route list above.

Thanks for your help Amy.

Reply to
Amy L.

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