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.