BGP process

Unanswered Question
Apr 16th, 2007


I have to run two BGP routing process on my 7204VXR with two diffrent AS numbers on same router,which IOS will it support?


I have this problem too.
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mohammedmahmoud Mon, 04/16/2007 - 08:55

Hi there,

A Cisco router can support only 1 BGP process. If you would like you can share us your scenario in order to think about a solution.


Mohammed Mahmoud.

Harold Ritter Mon, 04/16/2007 - 09:11


The local-as feature will indeed allow you to run two eBGP sessions to two different peers using two different AS numbers. You will still have only one BGP process though.

router bgp xyz

nei a.a.a.a remote-as abc

nei b.b.b.b remote-as cde

nei b.b.b.b local-as uvw

In this case, neighbor b.b.b.b would see your AS as uvw instead of xyz. Bear in mind that the xyz would also be part of as path seen by b.b.b.b. If you don't want xyz to be seen at all by b.b.b.b then you need to use the replace-as knob on the neighbor local-as command.

Hope this helps,

mohammedrafiq Tue, 04/17/2007 - 01:26


Thanks,I am having the issue at present as below,AS xyz is provided by my ISP.I need to change it to uvw,so I can have this peering with my existing ISP and new peering with my 2nd ISP through uvw, which is easy, but the issue come in because I am advertising 8 prefixes to my Crunnent ISP , out of 8 one Prefix is also provided by my ISP,and they only let me advertise that through xyz due to filters in place on their site.

Any suggestions welcome.


mohammedmahmoud Mon, 04/16/2007 - 09:13

Hi Mohammed,

Yes sure, but this feature was intended for migration scenarios not for normal running scenarios, try to design your network as optimum as you can:

"The BGP Support for Dual AS Configuration for Network AS Migrations feature allows you to merge a secondary autonomous system under a primary autonomous system, without disrupting customer peering sessions. The configuration of this feature is transparent to customer networks. This feature allows a router to appear, to external peers, as a member of secondary autonomous system during the autonomous-system migration. This feature allows the network operator to merge the autonomous systems and then later migrate customers to new configurations during normal service windows without disrupting existing peering arrangements."

"BGP prepends the autonomous system number from each BGP network that a route traverses to maintain network reachability information and to prevent routing loops. This feature should be configured only for autonomous-system migration, and should be deconfigured after the transition has been completed. This procedure should be attempted only by an experienced network operator, as routing loops can be created with improper configuration."

best regards,

Mohammed Mahmoud.


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