Route Redistribution: Why OSPF routes does not replace RIP routes at every location

[Steph1963]

Hi to All,

Just took my Troute class last week, and we had this very interesting example for route redistributing.

The LAB guide was good enough to predict that either R1 or R3 would have a routing table populated with RIP and OSPF route while the other routers would only have RIP route.

Even better, it predicts that if R1 would come up with RIP and OSPF routes, 172.16.1.3 route would be an OPSF route as apposed to RIP route. Playing with this network also showed me that if R3 would come up with RIP and OSPF routes, 172.16.1.1 route woud be an OSPF route as opposed to a RIP route.

Since the results predict in the lab can repeated steadily, I thouthg that this might indicated that a routing table that discover a non external route first could not be replaced by a route that is tag as external but that does not explain at all why routes like 172.16.1.1 are discovered via OSPF while the others are discovred as RIP.

Any help that could explain me why both routers (R1 & R3) does not only used OSPF routes since the administrative distance of OSPF is lower than the one used by RIP would be greatly appreciated.

Nobody in the class were able to figure what was the reason behind this, and I feel that there is something that I have not understand about routing.

R1#show ip route

172.16.0.0/32 is subnetted, 4 subnets

O E1 172.16.1.4 [110/22] via 10.1.101.99, 00:01:34, FastEthernet1/0

C 172.16.1.1 is directly connected, Loopback0

O E1 172.16.1.3 [110/22] via 10.1.101.99, 00:01:34, FastEthernet1/0

O E1 172.16.1.2 [110/22] via 10.1.101.99, 00:01:34, FastEthernet1/0

10.0.0.0/24 is subnetted, 6 subnets

C 10.1.13.0 is directly connected, FastEthernet2/0

C 10.1.12.0 is directly connected, Serial0/0.1

O E1 10.1.24.0 [110/22] via 10.1.101.99, 00:01:34, FastEthernet1/0

O E1 10.1.34.0 [110/22] via 10.1.101.99, 00:01:34, FastEthernet1/0

O 10.1.103.0 [110/2] via 10.1.101.99, 00:01:34, FastEthernet1/0

C 10.1.101.0 is directly connected, FastEthernet1/0

O E2 192.168.1.0/24 [110/20] via 10.1.101.99, 00:01:26, FastEthernet1/0

O E2 192.168.2.0/24 [110/20] via 10.1.101.99, 00:01:27, FastEthernet1/0

Thanks for your help

Stéphane

R3#show ip route

172.16.0.0/32 is subnetted, 4 subnets

R 172.16.1.4 [120/1] via 10.1.34.4, 00:00:16, Serial0/0.1

O E1 172.16.1.1 [110/22] via 10.1.103.99, 00:00:43, FastEthernet1/0

C 172.16.1.3 is directly connected, Loopback0

R 172.16.1.2 [120/2] via 10.1.34.4, 00:00:16, Serial0/0.1

10.0.0.0/24 is subnetted, 6 subnets

C 10.1.13.0 is directly connected, FastEthernet2/0

O E1 10.1.12.0 [110/22] via 10.1.103.99, 00:00:43, FastEthernet1/0

R 10.1.24.0 [120/1] via 10.1.34.4, 00:00:16, Serial0/0.1

C 10.1.34.0 is directly connected, Serial0/0.1

C 10.1.103.0 is directly connected, FastEthernet1/0

O 10.1.101.0 [110/2] via 10.1.103.99, 00:00:43, FastEthernet1/0

O E2 192.168.1.0/24 [110/20] via 10.1.103.99, 00:00:43, FastEthernet1/0

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hi,

   It's all about the timing of the routing protocol. On R3 if you do a "clear ip route 10.1.24.0 255.255.255.0"  command you might see this network learnt via O*E2 or RIP. Okay let's think of these steps.

1. You configure 2-way redistribution on R3 first.

2. You will see O*E2 for all routes,excluding connected routes at R1. This is correct.

3. You now configure 2-way redistribution on R1. There is no RIP routes on R1. How can R1 redistribute RIP routes into OSPF? So now R3 has not learnt O*E2 routes via R1.  R3 now shows some routes as RIP again.

  Good question!!! Guy

HTH,

Toshi

[Steph1963]

Hi Toshi,

It definitely helps.....

If I understand you correctly, the idea is simply that if the router did not learn RIP route because it learns OSPF routes first, RIP routes will never be distributed since they simply do not exists in the routing table.

Many thanks

Stephane

You are definitely a good answer man....

[Thotsaphon Lueangwattanaphong]

Hi Stephane,

    Your understanding is correct. It needs to be in RIB then the router will redistribute it.  Troute is fun,isn't it?

Many Thanks

Toshi