How to configure RIP on a Cisco router

Lessons Discussion
21

Hello Gaith

I understand where you’re coming from. I think it all has to do with the words being used.

To clarify, RIPv2 is a classless routing protocol. It performs CIDR, no question. It sends the subnet mask etc and supports VLSM. Now you mentioned in a previous post that:

I am in total agreement. What we are interested in for this specific instance is not so much the inherent workings of the routing protocol, but the end result. With the auto-summary enabled, the end result of the entry in the routing table in Router 1 of our example will be 10.0.0.0/8 which is indeed what we would get with RIPv1. This is something we do not want. To remedy this we use the no auto-summary command which changes the behaviour of the protocol so that we get the CIDR result we want. This I believe was the central idea behind Rene’s comment. It does not change the classless nature of the protocol, it just changes the behaviour so that we can take advantage and use this classless nature.

I hope this has been helpful!

Laz

22

hi Rene,
I am new to routing and studying your RIP topology. On your topology when looking at R1 routing table, it says 172.16.0.0/24 is subnetted, 1 subnets, but when I built the topology it says 172.16.0.0/24 is variably subnetted, 2 subnets, 2 masks, why the difference? I am using IOS ver 15.6(2)T

23

Hello Alain

The routing table will show what routes are available to the router to route packets. In the lesson, the statement “172.16.0.0/24 is subnetted, 1 subnets” indicates that there is only one /24 destination in the routing table that falls within the classful range 172.16.0.0 to 172.31.255.255.

In your routing table, if it states “172.16.0.0/24 is variably subnetted, 2 subnets, 2 masks” that means that you have some additional routes configured in the routing table. These may be from directly connected networks, that is, interfaces that you have configured on the device itself, or routes that have been learned via RIP. If you look at the subnets that are indicated, you can get an idea of where these have been learned from.

If you like, share with us your routing table in full so that we can see where those network destinations were learned from.

I hope this has been helpful!

Laz

24

Hello @lagapidis,

I find this statement to be incorrect:
"172.16.0.0/24 is subnetted, 2 subnets R 172.16.1.0 [120/2] via 192.168.23.2, 00:00:16, FastEthernet1/0" because we are subnetting 172.16.0.0/16 network not a /24.

Regards.

25

Hello sales2161

This is actually correct. When a dynamic routing protocol informs a router of multiple subnets that exist within a classful network that is larger than the subnet masks, and these subnets are all of the same size, the “/XX” shown in the title of the grouping will actually be equal to the size of all the individual subnets. I actually tested this out in a lab where I have two routers connected that are sharing routes via RIP. R1 is sharing the following subnets with R2:

  • 172.16.2.0/24
  • 172.16.4.0/24
  • 172.16.5.0/24

The resulting routing table on R2 is the following:

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 3 subnets
R       172.16.2.0/24 [120/1] via 10.10.10.10, 00:00:01, GigabitEthernet0/0
R       172.16.4.0/24 [120/1] via 10.10.10.10, 00:00:01, GigabitEthernet0/0
R       172.16.5.0/24 [120/1] via 10.10.10.10, 00:00:01, GigabitEthernet0/0

Notice that the title of the grouping shows /24 and not /16 which is the classful mask. If I change the subnet mask of one of the subnets to /25 and after waiting for the old route to time out, we get:

 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
R       172.16.2.0/25 [120/1] via 10.10.10.10, 00:00:21, GigabitEthernet0/0
R       172.16.4.0/24 [120/1] via 10.10.10.10, 00:00:21, GigabitEthernet0/0
R       172.16.5.0/24 [120/1] via 10.10.10.10, 00:00:21, GigabitEthernet0/0

You can see that the grouping now indicates /16, which is the classful mask.

Now keep in mind I’m using RIPv2. This same behaviour will be seen for directly connected routes as well, as long as there is more than one route that falls under the same classful range.

You can find out more about the structure of the routing table at this Cisco documentation.

I hope this has been helpful!

Laz

26

Hello @lagapidis,

Thanks a lot for an insightful reply. What do you think about this behavior? Do you find it to be helpful or it just creates confusion?

Regards.

27

Hi @sales2161,

I think this output is confusing, especially when it looks like this:

     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, FastEthernet0/0

On IOS 15, the output is different:

R1#show ip route 

Gateway of last resort is not set

      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/1
L        172.16.1.1/32 is directly connected, GigabitEthernet0/1
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, GigabitEthernet0/2
L        192.168.12.1/32 is directly connected, GigabitEthernet0/2

It now shows 172.16.0.0/16 as “variably subnetted” together with the network address and IP address. This output is easier to read.

Rene

29

Hi guys,

When you configured RIPv2 you were using the command no auto-summary which then treats it as classless thereafter.

Is RIPv2 classful by default until you use the no auto-summary command?

30

Hello Joseph

By default, RIPv2 will automatically summarize routes, just like RIPv1. When no auto-summary is used, then the protocol can send routing information across classful network boundaries. So in other words, yes, RIPv2 by default is classful.

I hope this has been helpful!

Laz

1 Like
31

makes perfect sense, Thanks Laz

1 Like