Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

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

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

7 REPLIES
Cisco Employee

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

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,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

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

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.

Cisco Employee

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

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
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

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

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

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

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.

Cisco Employee

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

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.

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

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

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

92
Views
0
Helpful
7
Replies
CreatePlease login to create content