Forcing a single subnet out of one BGP peer

[Christos Papageorgopoulos]

Hi all,

We have /18 network wich is announced on 3 different bgp peers. Selection of path is made based on the weight,local-pref., etc.

It is possible to forcing a single subnet /24 to choose one specify ISP transit for outgoing traffic without impacted the other subnet preference. Otherwise to

influencing routing based on source address rather than destination address

I dont think is it possible but i'm asking to be sure.

Thanks,

Christos

[stephen.stack]

Hi

At a very simple level, if you advertise the single /24 to only one peer, then Longest Prefix Match kicks in and when a packet arrives on an upstream router with both prefixes in the routing table, if the packets destination matches the /24 it will be routed via that preffered peer.

http://en.wikipedia.org/wiki/Longest_prefix_match

You will only need to ensure you are only advertising the prefix to that paticular peer, perhaps using route-maps, and you will not have to adjust any BGP metircs or polices such as Local_Pref or MED.

Regards

Stephen

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[Christos Papageorgopoulos]

Hi Stephen,

This is for the incoming traffic but my question is to choose any destination for a local subnet through a specify BGP peer.

Thanks

[John Blakley]

Christos,

You can use pbr to do this. Create an acl to match on the source subnet, create a route-map to match on the acl and set the next hop to the provider that you want to use. Anyone that doesn't match on the policy will use the normal routing table to route out.

Or are you wanting to advertise a /24 to just a single ISP? I guess I'm not understanding what you're wanting to do...

HTH,
John

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[stephen.stack]

Maybe i read the original Post incorrectly, but for outbound, yes PBR will work, or Local_Pref applied to the inbound /24 prefix using a route-map will work for traffic in the outbound direction from your AS.

Regards

==========================
http://www.rConfig.com 

A free, open source network device configuration management tool, customizable to your needs!

[Christos Papageorgopoulos]

Hi John,

So , i should to configure pbr on the interface which is attached the subnet /24. But in case of that peer fails what must be done so traffic from this /24 fall back to other peer?

Thanks,

Christos

[John Blakley · Accepted solution]

You could try it a couple of ways. You could set a next-hop and then a default next-hop in your route-map. You could also configure tracking and then track the route:

http://www.cisco.com/en/US/tech/tk364/technologies_configuration_example09186a0080211f5c.shtml

http://www.cisco.com/en/US/tech/tk364/technologies_configuration_example09186a00801f3b54.shtml#caseone

HTH,
John

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[Christos Papageorgopoulos]

Thanks a lot john and Stephen!