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New Member

OSPF (edge) and IBGP/MPLS (core) issues

I found a strange problem on a network and I'm not sure if it's normal behaviour or a bug. I have a customer running OSPF at their sites and connecting to an MPLS VPN as their core. They have their HQ site with dual links and their DR site with dual links. The four links are configured with the correct costs and weightings for the routes and links.

All spoke sites use the primary HQ link as their preferred path, then the backup HQ, primary DR, backup DR. The requirement is that the DR site also route to HQ site as it's preferred path instead of using it's local routes. The routes are hitting the DR PE correctly via IBGP and OSPF at the same time which is correct. Now if you look at the BGP routes it's preferred path is via the HQ. If you look at the OSPF routes it's preferred path is via the HQ. The problem is in the VRF routing table when the route is installed it's using the OSPF route over IBGP route because of the 110 and 200 administrative distances.

I added "distance 210" to the OSPF VRF routing process for the two DR PE's which I thought would make BGP more "believable". It did this but did something else which I can't explain. OSPF stopped receiving routes from it's DR neighbour. Do the distances need to be the same throughout the OSPF domain? I didn't think so. I then added the same distance command to all OSPF processes at the DR site. I started receiving routes again at the PE.

The next strange thing is BGP stopped redistributing it's routes into OSPF. My only explanation is that BGP won't redistribute into a protocol which has a less preferred AD than itself. Does this make sense? Can anyone confirm this or is this a bug?

My next step is going to be removing OSPF off the WAN links at the DR sites and run EBGP right up until the CE. With an AD of 20 and 110 the network should do what I want it to. It's just strange that my other fix didn't work. Does anyone know what the story is there?

Thanks,

Mike

1 ACCEPTED SOLUTION

Accepted Solutions

Re: OSPF (edge) and IBGP/MPLS (core) issues

Hello,

did you have a look at the option to setup OSPF sham links? This works like an intra area link through the MPLS cloud. Thus path selection with a sham link in place boils down to mere OSPF link metric adjustments.

For further reading and a more detailed explanation please consult:

"OSPF Sham-Link Support for MPLS VPN"

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087ce2.html

Hope this helps! Please rate all posts.

Regards, Martin

4 REPLIES
New Member

Re: OSPF (edge) and IBGP/MPLS (core) issues

Have you added the backdoor command on the necessary router

New Member

Re: OSPF (edge) and IBGP/MPLS (core) issues

Mike,

If I understand right, here is the simplfied version of your issue.

Topology: CE_HQ-(ospf)-PE_HQ-(ibgp)-PE_DR-(ospf)-CE_DR. Another private connection: CE_HQ-(ospf)-CE_DR.

All the traffic between CE_A and CE_B is through the private connection, but you prefer it through PE routers (MPLS).

Your plan worth a try. The problem is how you stop PE_HQ from learning all the routes from ospf. You might need to change HQ site to ebgp as well.

Please post your test result.

Thanks.

Re: OSPF (edge) and IBGP/MPLS (core) issues

Hello,

did you have a look at the option to setup OSPF sham links? This works like an intra area link through the MPLS cloud. Thus path selection with a sham link in place boils down to mere OSPF link metric adjustments.

For further reading and a more detailed explanation please consult:

"OSPF Sham-Link Support for MPLS VPN"

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087ce2.html

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: OSPF (edge) and IBGP/MPLS (core) issues

Brilliant :)

That's exactly what I was trying to do.

Thanks,

Mike

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