Configuring Totally Stub Areas without Losing Connectivity

Answered Question
Aug 2nd, 2010

I have an ABR with the standard area 0 and area 1. In area 1, there are about ten remote sites which I would like to configure as totally stub areas.

When I configure the remote site and ABR for area 1 "stub no-summary", all the other remote sites will drop their adjancies. I will not be able to access those remote sites to add the "stub no-summary" statement. Is there a strategy to do this without losing connectivity? Thanks in advance.

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Correct Answer by west33637 about 6 years 4 months ago

Hello David. To configure area 1 as totally stubby, every interface in area 1 must be configured as stub. However, you only need to configure area 1 stub no-summary on the ABR. all the other routers with interfaces in area 1 should be configured as area 1 stub. not area 1 stub no-summary.

Im assuming your pc is in area 0 and that is why you get locked out of the area 1 links when you configure totally stubby on the ABR and remote link.

Here is what you can do to fix this issue.

initially configure the ABR interface as area 1 stub. This will allow LSA type 3 and 4 networks into area 1 and that way you should not lose connectivity to the other area 1 links.

Then configure every router with links in area 1 as area 1 stub, not area 1 stub no-summary. Only the ABR needs to be configured as no-summary.

When you have completed this, then go on the ABR and configure area 1 stub no-summary. This will make the entire area 1 totally stubby.

The ABR will now originate a default route in place of LSA type 3, 4 and 5 advertisments.

Please remember to rate this post if it helps.

Correct Answer by Richard Burts about 6 years 4 months ago

If we knew more about the topology of the network we might be able to give somewhat better advice. But 2 thoughts come to me as I think about your question:

- it might help to start with the router that is furthest away from the ABR. Configure that router as totally stub. It will drop its neighbor relationship, but its neighbor should still be reachable from the ABR so you can access it and configure it as totally stubby. Work your way from the furthest away to the nearest. You eventually wind up with all routers in area 1 configured as totally stubby and all neighbors again. (obviously this gets more complex if your PC happens to be in area 1) While this should work I recognize that it does not meet part of your criteria about not losing connectivity.

- if it is really important that you not lose connectivity then I would suggest a different approach. Configure static routes in area 1 that duplicate the results of the OSPF routes in area 1 and redistribute static into OSPF. While the routers are configured consistently (all normal or all totally stubby) the OSPF intra area routes will be preferred over the external redistributed static. As you reconfigure routers to be totally stubby they will drop their OSPF neighbor relationship and their OSPF routes will be withdrawn from the routing table which will allow the redistributed static route to be used. As the process is completed and all routers are totally stubby the OSPF intra area routes will again be in the routing table and you can remove the static routes and the static redistribution.

HTH

Rick

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Correct Answer
Richard Burts Mon, 08/02/2010 - 11:24

If we knew more about the topology of the network we might be able to give somewhat better advice. But 2 thoughts come to me as I think about your question:

- it might help to start with the router that is furthest away from the ABR. Configure that router as totally stub. It will drop its neighbor relationship, but its neighbor should still be reachable from the ABR so you can access it and configure it as totally stubby. Work your way from the furthest away to the nearest. You eventually wind up with all routers in area 1 configured as totally stubby and all neighbors again. (obviously this gets more complex if your PC happens to be in area 1) While this should work I recognize that it does not meet part of your criteria about not losing connectivity.

- if it is really important that you not lose connectivity then I would suggest a different approach. Configure static routes in area 1 that duplicate the results of the OSPF routes in area 1 and redistribute static into OSPF. While the routers are configured consistently (all normal or all totally stubby) the OSPF intra area routes will be preferred over the external redistributed static. As you reconfigure routers to be totally stubby they will drop their OSPF neighbor relationship and their OSPF routes will be withdrawn from the routing table which will allow the redistributed static route to be used. As the process is completed and all routers are totally stubby the OSPF intra area routes will again be in the routing table and you can remove the static routes and the static redistribution.

HTH

Rick

Correct Answer
west33637 Mon, 08/02/2010 - 12:50

Hello David. To configure area 1 as totally stubby, every interface in area 1 must be configured as stub. However, you only need to configure area 1 stub no-summary on the ABR. all the other routers with interfaces in area 1 should be configured as area 1 stub. not area 1 stub no-summary.

Im assuming your pc is in area 0 and that is why you get locked out of the area 1 links when you configure totally stubby on the ABR and remote link.

Here is what you can do to fix this issue.

initially configure the ABR interface as area 1 stub. This will allow LSA type 3 and 4 networks into area 1 and that way you should not lose connectivity to the other area 1 links.

Then configure every router with links in area 1 as area 1 stub, not area 1 stub no-summary. Only the ABR needs to be configured as no-summary.

When you have completed this, then go on the ABR and configure area 1 stub no-summary. This will make the entire area 1 totally stubby.

The ABR will now originate a default route in place of LSA type 3, 4 and 5 advertisments.

Please remember to rate this post if it helps.

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