I'm a bit new to BGP and need a little assistance with a configuration change. We are currently using our MPLS connection for all traffic to all sites. Going forward We would want to segragate the Datacenter backup link from the other spoke sites and will be adding an additional MPLS leg in our core datacenter to accommodate. See image of current setup and router configs. Diagram before new service Router Configurations

I have ordered a new MPLS leg with a Closed User Group(CUG) from the ISP. This new service is for datacenter to datacenter traffic only. The existing MPLS leg will be used exclusivly for our remote sites.(Hub-Spoke) enter image description here enter image description here

My question is how do I program the core router with this new CUG that has the same AS number from the ISP (AS207). Do I just add the new network and neighbor to the existing AS 65001? I believe I only need to change the core router because the backup DC MPLS will be integrated into the new CUG.

This is a production network which is why I'm a little apprehensive making this program change.

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    The other sites should not be able to reach the backup DC? If you add the network under BGP it gets advertised to all peers unless you do filtering. This may or may not be a problem depending on what you are trying to achieve.
    – Daniel Dib
    Nov 11, 2014 at 7:15
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    It would be fine for the spoke sites to have the routes to the backup DC. I just need traffic between Datacenters to use the CUG link exclusivly. Conversly, I need the Core DC - Spoke sites to use the original MPLS link exclusivly. The reason for the rigid path selection is that we are using Riverbed Steelhead WAN excellerators to enhance the links. One Riverbed pair between DC's and another Riverbed at the core for the spoke sites. I could have enabled Policy based routing with a single MPLS link but that was going down a road I didn't want to travel or support long term. Nov 11, 2014 at 17:17
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    – Ron Maupin
    Jan 4, 2021 at 1:43

1 Answer 1


There a few inconsistencies in your diagram / configurations. In the diagram it is AS 207, in the configuration it is AS 209 and in the final configuration it is AS 207.

If the provider AS is the same in both MPLS networks then:

  • I'll call your original network CUG-1 and your new one CUG-2.

  • There won't be any problems with advertising the routes to CUG-2 as it will be in a different VPN to CUG-1. If you don't do any filtering then the routes will be automatically advertised to CUG-1 via the core (provided you aren't separating them with VRFs on that device). You will need to rewrite the AS number before advertising it out to the spokes from your core and vice versa.

If you don't do this then:

  • Backup DC routes will be dropped by the CUG-1 PE router due to the BGP loop prevention mechanism i.e. routers in AS207 will see their own AS in the AS-Path ( e.g. AS 65002 -> AS 207 -> AS 65001 -> AS 207 DROP).

  • Spoke routes will be dropped by the CUG-2 PE router for the same reason (AS 65004 -> AS 207 -> AS 65001 -> AS 207 DROP).

If you use BGP AS-override then the AS-Path will look something like this on e.g. Backup DC to Spoke Site: AS 65001 -> AS 65001 -> AS 65001 -> AS 207 -> AS 65004.

Because you will be circumventing the BGP loop prevention mechanism, ensure that the Core DC is the only location that is peering with both CUGs.

Wrote this on mobile so let me know if it is a bit disjointed and I'll clean it up later.

  • Yea, I noticed the AS numbering after I posted the question. The provider AS number is same across all devices. Good catch. The core is the only location that will be peering with all locations. That is correct. Nov 12, 2014 at 19:20

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