Hi, rather than memorize what you have to do, you may want to break it down so that you understand the method behind the madness, so to speak.
If you have a router that acts as a "go-between" for a RIP network and an OSPF network by doing mutual redistribution, you have to ensure that the OSPF routes that it learned from the OSPF Router are not re-learned and redistributed back to the OSPF router when the RIP router readvertises them as RIP routes.
For example, refer to the attached diagram. SEE NEXT POST.
Router A is a RIP router, Router B runs RIP and OSPF and router C is an OSPF router.
Router A advertises RIP networks 1-10 and Router C advertises OSPF networks 11-20.
Lets start with the OSPF router's interaction.
Router B will learn about OSPF routes 11-20 from OSPF Router C and then advertise them to Router A as if they were RIP routes (redistribute OSPF into RIP), right?
OK...
So RIP Router A will learn about OSPF networks 11-20 and enter them into its routing table as RIP networks. The next hop for networks 11-20 will be Router B, since it learned them from B.
Now comes the problem.
RIP Router A will then readvertise them to Router B 30 seconds later. If Router B does NOT filter the routes for networks 11-20, they will be readvertised back to the OSPF Router with reachability information (next hop) that will again point to Router B.
The OSPF process running on OSPF router C will think it has a valid route for what is really a network in the OSPF domain that has a next hop that points in the direction of an external system (Routers A).
HTH
Victor
