Wednesday, December 24, 2008

IE Vol 1 OSPF Domain-id complete

This was another interesting lab. Typically in MPLS, redistribution is done from OSPF to BGP and back to OSPF in order to connect OSPF together across the P network. Because of the redistribution, OSPF sees the remote sites as External (LSA type 5) networks.

But since these remote sites are likely to be within one company's control, having them marked external may not be the best option. Instead, the domain-id can be used on the PE router under the ospf process. When the PE router advertises the ospf route to the CE router, it looks at the domain ids of both routes. If they match, the ospf route is advertised as an inter-area (IA) route instead of an External route.

This seems like it would be important for a couple of reasons. IA routes are always preferred over E routes. Also, E routes are not advertised into stubby areas, while IA summaries are. In more complex scenerios with a mix of nssa areas and redistribution, a company may prefer to use its provider's MPLS network rather than another non-MPLS link. The domain-id allows for this much easier than filtering routes from the edges.

1 comment:

tedhi said...

Since they`re using process-id on PE-CE router, the route-target import need more attention on the provider perspective.