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New Member

BGP question?

Can I have 2 different ASs, say AS 65001 and AS 65002, connected to same OSPF domain? So that both ASs advertise the routes from the OSPF domain. So say you have two routers one in AS 65001 and one in 65002, both of these routers connect EBGP to AS 65003 and both of these routers have a connections to the same OSPF domain that supports the network. Thank you for your help.

Hall of Fame Super Silver

Re: BGP question?


While you could physically configure one router to run BGP 65001 and the other router to run BGP 65002 while connected to the same OSPF network it would not be a good thing to do. It violates several expectations of BGP. You do not mention in your question whether these routers will connect to each other. If they do connect then the connection should be IBGP since they appear to be in a common enterprise (since they share the same OSPF). But their connection would be EBGP since they have different AS numbers. Also the router in 65003 would see the same network(s) (from OSPF) as originating from 2 different AS which is not a good thing in BGP.

If we knew what you were trying to accomplish with this we might be able to suggest a better way of accomplishing the objective.



Re: BGP question?

Rick, you say, "If they do connect then the connection should be IBGP since they appear to be in a common enterprise (since they share the same OSPF). But their connection would be EBGP since they have different AS numbers."

Can't their BGP processes be configured so as to identify each other as neighbors with the same AS number (i.e., whichever AS number is assigned to the OSPF domain (Franklin didn't specify if that's 65003 or a fourth AS)?

And then they could still form additional IBGP peerings within their respective 65001 and 65002 AS'es, couldn't they?

New Member

Re: BGP question?


The routers in AS 65001 and 65002 are not connected to each other. They can only connect to 65003? This is for a project we are testing and I am wondering if this confuguration will work. I am sure the router in 65003 will see both routes, no tested yet because the lab is being used by someone else as we speak, but I am trying to understand the harm in this configuration. The 65003 router will be get the same route from two ASs and pick one according to the BGP decision making algorithm. I guess I cant get my head around the harm of the 65003 router seeing both routes. Thank you for your help.

Re: BGP question?

Franky-baby, it really would help if you could post a drawing or at least give some more specific about your topology.

You said the OSPF domain supports the network - is that the only place you have subnets that belong to that network? Are there no other major networks or their subnets in play there? If so you might consider advertising a default route for the 10./8 Class A network into the other AS's rather than have BGP eat up your CPU cycles.

Re: BGP question?


I don't think that would be an issue on the 65003 router. It should be fine to see the same routes from different EBGP peers. BGP will normally pick the best route to install in RIB. If you want AS 65001 and AS 65002 to talk with each other. It should be a problem.

Edit: It would be good if you can provide us how they are connecting and which network you're using.


Re: BGP question?


Yes. You can. Is this for testing? OSPF and BGP can run together on the same router. BGP is not autometically advertise networks. You have to use available methods to do so.

Note : If this was not the answer. please clarify about the question in detail.


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