ospf on Inter-AS back to back vrf links

This started out as one of the INE labs, practice for Inter AS topology with back to back vrf's. One of the vrf's ran ospf on each step in the path and didnt work correctly, in that I was unable to get ospf cutomer routes from one vpn into the other.

With some troubleshooting I realised that the design I had created was not valid, it is as follows:

R1(CE) - R2(PE) VPN 1 R3(PE) - R4(CE) Back to back VRF R6(CE) R7(PE) VPN 2 R8(PE) - R9(CE)

Each CE-PE link is in area 0 and the same domain-id (process id)  is used on each router.

The back to back CE routers learn all the routes from their respective VRF.

I now know that this shouldn't work, its a bad design but so far have been unable to find a description of the ospf process that prevents this from working.

I fixed it by running eBGP for the back to back VRF link, but this didnt really aid my understanding of why its a bad design and what mechanism prevents the routes getting injected from one vpn to the other.

 

Andy

 

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