2 AS and 2 INet connetions. Need help with traffic flow.

[dumpeal]

I have 2 Autonomous systems in this network that each have a router that provides a connection to the Internet. The Autonomous systems are seperated by a frame switch.

I want to get hosts and routers that are located on an AS going out to the Internet from the router in that AS and not traverse to the other. For some reason, all the traffic from 1 AS keeps crossing the frame switch to go out the other router that goes to the Internet.

We're running OSPF over frame-relay if that helps on this network. If you've got any tricks or ideas, it would be most appreciated.

Thanks

[Harold Ritter]

A couple of questions.

- Do you really have 2 ASNs?

- What routing protocol are you running between these two ASes?

Could you give us a bit more information about the topology.

Thanks,

[dumpeal]

Hey, hritter

-Yes, we really do have 2 AS.

-BGP is ran between the 2 AS while a few OSPF areas are made in each AS.

Hope this helps.

[Harold Ritter]

This makes it easy then. Using a route-map, you can set the local preference lower for prefixes originated by the peering AS and set the local preference higher on other prefixes. This would cause traffic destined to prefixes in the peer AS to use the directly conected link to the other AS and all other traffic to be forwarded toward the respective ISP.

[Harold Ritter]

I'm sorry, I meant local pref higher for prefixes originated by the peer AS and local pref lower on all other prefixes.

[dumpeal]

Hey hritter,

Thanks for the reply. I'm not exactly sure how one would go about setting these prefixes. Are they put on a particular router? If you could give me some ideas that would help, I'm going to do some more research on this.

[Harold Ritter]

On the routers connecting the two ASes, you could configure someting like this:

route-map SetLocalPref permit 10

match as-path 1

set local-preference 110

route-map SetLocalPref permit 20

set local-preference 90

ip as-path access-list 1 permit ^x$

router bgp x

nei route-map SetLocalPref in

where x would be the peer ASN.

This would set a local-preference of 110 (better than default 100) for prefixes originated by the peer AS and a local-preference of 90 (worst than default 100) for prefixes received from peer but not originated by peer (Internet).

For this to work you have to make sure that you run iBGP between the Internet facing router and the peer AS facing router (assuming they are two separate routers).

You should also make sure that you are filtering prefixes you advertise to your ISP not to include the non locally or non peer originated prefixes. Omitting this might cause your AS to become a transit AS.

[dumpeal]

Awesome, thanks for this info. I'm still an amateur when it comes to BGP configs but this seems to make some sense.