Basic route redistribution question

martin.ray · ‎04-26-2009

I know how to configure it, but I just want to ask a basic question about it and what I have configured.

Say I redistribure OSPF into RIP

router rip

redistribute ospf 2

I have set the OSPF redistribution into a RIP metric with 2 hops. Can someone go into detail about this '2 hops' ?? Is it 2 hops to go and the route will then die, or is it 2 hops back to the OSPF network ????

lamav · ‎04-26-2009

The route table will show that the network you redistributed into OSPF has a 2 hop metric to the destination. The route doesnt die, so to speak. Setting the metric is just for the purpose of manipulating routes.

So, if you redistrubute into RIP and set a metric 10, the RIP router that is directly connected to the redistribution router will receive a route for the destination network with a hop mnetric of 10. But that doesnt mean the network is really 10 hops away. If that RIP rouuter only had that route and placed it in the routing table, it would use it, and, as you know since you did the redistribution, it is not 10 hops away, but only 1.

Makes sense?

Victor

martin.ray · ‎04-26-2009

Ouch, this is gonna hurt my brain !!!

So the redistributed hop count has not the same effect as standard hop counts whereas a packet dies when it reaches it limit.

lamav · ‎04-26-2009

Martin:

Imagine you have 3 routers, R1, R2 and R3.

R1 is an OSPF router.

R2 is OSPF and RIP

R3 is RIP

At R2, I redistribute R1's loopback interface into RIP and give it a metric of 10.

R3 will receive that RIP route and have a metric of 10.

Is the loopback interface really 10 hops away? No.

R3 only knows what it is told by R2. It does not have visibility beyond that redistribution point. In fact, even if it was another RIP router that advertised a directly connected route as being 10 hops away, it would believe that, too.

This is why they call distance vector routing "routing by rumor." All R3 knows is what he is told by R2. He does not have visibility to the overall topology, as OSPF/link state protocols do.

HTH

Victor

martin.ray · ‎04-26-2009

Excellent, I got it.

Thanks for the help Victor.

lamav · ‎04-26-2009

You got it, Martin. :-)

Victor