ROUTE Notes - OSPF Filtering and Summarization

Sun, 20 Jun 2010 00:00:00 +0000

Feel free to correct all this stuff. Additions are also welcome.

Study Questions

How do I keep an area route from reaching a router in that area?

You don’t. That defeats the whole purpose of having the topology database on every router. If you filtered one route from a router, there’s no way that SPF could calculate routes correctly.

Fine, then. Where do I filter routes?

You filter routes on an ABR or ASBR. Since routers only have the whole topology for their area, it’s safe to filter routes from another area or from a redistributed routing protocol. On a more technical note, you’re filtering type-3 LSAs on an ABR and type-5 LSAs on an ASBR.

ROUTE Notes - Controlling Routes in EIGRP

Thu, 17 Jun 2010 00:00:00 +0000

Corrections welcome.

Study Questions

Why would you ever want to summarize routes?

Summarizing routes minimizes the routes advertised to the network. For example, instead of advertising 192.168.0.0/24, 192.168.1.0/24…192.168.n.0/24, a router can advertise a single route to 192.168.0.0/16. Keeping routing tables small saves hardware resources, minimizes convergence times, helps avoid route flapping, and makes the routing table easier to read for humans.

When will an EIGRP router auto-summarize a route?

If a router has interfaces that that are in different classes of network (Class A, B, C), then that router will auto-summarize those routes up to the classful boundary. For example, if you have a 10.0.0.1/24 and a 192.168.100.1/30, the router will advertise 10.0.0.0/8 and 192.168.100.0/24.

Summarization on Aaron's Worthless Words

ROUTE Notes - OSPF Filtering and Summarization

ROUTE Notes - Controlling Routes in EIGRP