OSPF inter-area vs intra-area routes

[tech]

Is there a config that would allow me to prefer an inter-area route over an intra-area route to the same network? Then when the inter-area route is not available, the intra-area route would be injected into the routing table.

I can use a "distribute-list route-map in" config to suppress the intra-area route intially allowing the inter-area route to be placed in the routing table. The problem I see is the ability to inject the intra-area route into the routing table once the inter-area route is no longer available.

Thanks,

RJ

[Kevin Dorrell]

I'm not sure what you are trying to achieve by doing this, but you could try distance ospf inter-area 109 inside your router ospf block.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800917e6.html#wp1017956

Kevin Dorrell

Luxembourg

[Richard Burts]

Kevin has come up with an interesting possible solution. I am not clear whether the original post is looking for something that will prefer inter-area routes over intra-area route for the same network for ANY route (which is what Kevin's solution does) or is looking for this for a specific route.

If you try the solution suggested by Kevin please let us know how it worked.

HTH

Rick

[tech]

Thanks for the post, Kevin.

I am trying to prefer all inter-area routes over intra-area routes in this case, not just a specific network.

I tried distance ospf intra-area 120 and the intra-area routes were still in the routing table even though the inter-area routes had a 110 distance value. I have not tried making the inter-area routes AD lower than the intra-area routes as you suggest. I believe this will have the same result as my previous attempt that I mention above. My understanding of the distance route type command is that it will only work with multiple OSPF processes not within the same OSPF process. I will give your suggestion a a try later on today.

Thanks,

RJ

[Richard Burts]

RJ

The more I think about it the more I believe that you are correct that the distance command would influence choice of routes from different OSPF processes but will not influence the choice of routes within a single OSPF process. The administrative distance is used when the routing protocol process (like OSPF) nominates a route to the process that controls the contents of the routing table. But distance is not used withing the routing protocol itself.

Within a single OSPF process I believe the decision is based on the OSPF specification which says that intra-area routes are always preferred over inter-area routes to the same prefix.

HTH

Rick