Still confused about RIPv1

workfrom925

I still don't get RIPv1 very well. If it's classful routing, then how does it know 209.165.200.228 has /30 subnet mask? Why doesn't it use /24?


R3#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

R 10.0.0.0/8 [120/1] via 209.165.200.233, 00:00:07, Serial0/0/1
172.30.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.30.100.0/24 is directly connected, FastEthernet0/0
C 172.30.110.0/24 is directly connected, Loopback0
C 172.30.200.16/28 is directly connected, Loopback1
C 172.30.200.32/28 is directly connected, Loopback2
209.165.200.0/30 is subnetted, 2 subnets
R 209.165.200.228 [120/1] via 209.165.200.233, 00:00:07, Serial0/0/1
C 209.165.200.232 is directly connected, Serial0/0/1
R3#

chX

This is a bit hazy and a bit of a shot in the dark, but I think this is because you have a directly connected network in the same major class. I'm talking about:

C 209.165.200.232 is directly connected, Serial0/0/1

I recall reading that if you have a directly connected network in the same major class as a prefix learnt by a classful protocol, it will just use whatever subnet mask you've applied on the interface for that directly connected network.

Could be way off, but that's my first guess!

NetworkVeteran

chX wrote: »

I think this is because you have a directly connected network in the same major class.. Could be way off, but that's my first guess!

You're right on, and I've pointed this out to him before.

d6bmg

chX wrote: »

This is a bit hazy and a bit of a shot in the dark, but I think this is because you have a directly connected network in the same major class. I'm talking about:

C 209.165.200.232 is directly connected, Serial0/0/1

Yes, it is due to that connected network.

chX

Good to know. It's reassuring to know that I can remember a detail like that from so long ago, and yet I easily forget all about the OSPF area types and stub/NSSA stuff!

workfrom925

NetworkVeteran wrote: »

You're right on, and I've pointed this out to him before.

What if that other link is not /30? Ripv1 would apply /30 anyway since the directly connected 209.165.200.232 has /30? It wouldn't be correct then.

What if there is another directly connected 209.x.x.x network with a subnet mask of /24? Would Ripv1 apply /30 or /24 to the other network?

NetworkVeteran

workfrom925 wrote: »

What if that other link is not /30? Ripv1 would apply /30 anyway.. What if there is another directly connected 209.x.x.x network with a subnet mask of /24? Would Ripv1 apply /30 or /24 to the other network?

Applying more than one subnet mask to the same network is a recipe for trouble, because while RIPv1 supports subnetting, it does not support VLSM or discontiguous network designs. Try labbing it up and see what happens.