How to peer with two un-related routers which are using the same BGP AS

[andrew_ho]

Hi,

I ran into a situation where I have to configure a router to peer with two un-related routers (belong to two different organizations) using the same private AS.

Router A (AS 64514)  --- Router B (AS 65007) --- Router C (AS 64514)

I have control of Router B&C but not A. All the AS cannot be changed.

Any suggestion on how to get this working?

Thanks,

Andrew Ho

[andrew_ho]

Hi,

Forgot to mention that routes need to be exchanged between router A and C.

Andrew

[cisco_lad2004]

Have you looked at allow as in feature ? I think its what you need.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080b59d08.shtml

HTH

Samir

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[sreenath1987]

I don't think there should be any issue .

Get the basics right, you need connectivity between two routers A and C which are two different offices.. So they will not have any physical connection.. Hence, ensure that you form a routing path, may be GRE, then try forming the neighbor relation.

I have not tried this, but should be okay. Let us know what's the exact issue you face

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[cisco_lad2004]

RouterB peers with A & C, since they have same AS, prefixes would be rejected. Allow AS in will bypass this default behaviour.

There was no mention of direct peering...which I cant see how to establish before routing between A & C in 1st place.

HTH

Samir

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[andrew_ho]

Hi All,

In fact, I am looking for the "as-override" feature but in a non-mpls setup. I have tried on IOS 12.4 and 15.0 but cannot find this command? BTW, I have tried on a Cisco 1941. Is this feature specific to particular model or feature set? Any idea or other suggestion?

Regards,

Andrew Ho

[Peter Paluch]

Andrew,

The as-override function is, to my knowledge, available in per-VRF setups only. However, as Samir suggested a few posts earlier, another approach is to use the neighbor allowas-in - do you believe this would work for you?

Best regards,

Peter

[andrew_ho]

Hi Peter,

The actual situation is more complicated than this, where router A & C are also indirectly connected via some other AS in between forming a loop, or redundancy if you wish.

I did try the allowas-in approach, it worked. However, it won't be able to handle some scenarios. Also, I don't have control of some of these external AS, making it difficult to implement.

As a result, the as-override approach would be ideal for me. But just can't find that command anywhere.

Regards,

Andrew  

[Peter Paluch]

Andrew,

You would find the as-override command if you configure BGP neighbors in a VRF:

router bgp 1

  address-family ipv4 vrf ABCD

    neighbor N.N.N.N remote-as X

    neighbor N.N.N.N as-override

However, that would require you to run your BGP on the "PE-like" device in a VRF. It is not necessarily tied to MPLS, i.e. this can be used outside any MPLS deployment. The need to place your routes in a VRF would probably complicate your setup, though.

Best regards,

Peter

[Jose Jara]

Another option is to re-advertise the prefixes. You may do summarization with aggregate-address without as-set. I had the same situation doing a design 1 year ago. In a migration of a big customer to MPLS/VPN, the customer was using the same provider in the domestic national network that we were using to interconnect via international to our cloud. So, the scenario was: AS 1 (our IP/MPLS AS) AS 2 (Provider) AS 3 (Our CPE) AS 2 (Provider) --Customer Network.

What I did was to do an aggregate-address summary only in Our CPE, without the as-set, in the upstream direction towards our Network. In the downstream direction I sent just the default route from the CPE, conditionally as long as we were receiving the default route from our network: Neighbor X.Y.Z.Y default-originate route-map DEFAULT.

Other options, depending of how many prefixes you are receiving (if not would be a lot of configuration...), would be to do an inject-map, create static routes and do conditional advertisement, etc...

Hope this helps,

JJ