classful routing examples

[SJ K]

Hi all,

I came across this term in my old textbook

"If a router running RIP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is called classful routing)"

I understand the concept of network/host , classful network, and subnetting, I also understand the concept of classless network and VLSM. But I have never use a router before.

Q1) In the sentence above, does it means that there is only 1 parameter/place in the router to set the overall subnet masks for all the interfaces in router that is running in RIP mode ?
 

Q2) How about in router running RIPv2 ? How would it be like ? For each interface, I am able to set its subnet mask of variable length to represent the network that the interface is connected ?
 

Would appreciate if someone gurus here can show me an example ?
 

Thanks.
Regards,
Noob

[Cisco Freak]

Jon,

This is going crazy!

Consider the this setup.

R1<->R2

What I see is that when I advertise a new network from R1, and if that new network has got the same subnet mask as the network that connects R1 and R2, then the new network will be advertised. Let me elaborate:

R1 and R2 connected with 172.16.1.0/24 subnet. I add the new loopback 172.16.2.0/24 in R1. The new subnet will be advertised to R2 as 172.16.2.0. That what we have seen in the capture I attached.

Suppose if I change the subnet mask of that loopback to something else other than /24, then it will not advertised. This is your observation.

CF

[Jon Marshall]

CF

Your test is what I would expect.

If the subnet mask is the same then it should advertise it.

If the subnet mask is different but it is part of the same major network as the one used to connect the routers then in the test I did it wasn't advertised.

I used debugging to see what updates the routers were sending and receiving and it's not that the route is sent but not accepted, the route is never actually sent.

But you did an earlier test that suggested a route was being sent.

Perhaps I misunderstood what you were saying in your earlier test ?

Jon

[Cisco Freak]

Jon,

It was the same subnet that was sent, it was part of the same major subnet. I am still not able to figure why it showed a /32 route. I think it could be a GNS3 bug. Because I remember some time back when I tested it, it worked as expected.

I think I caused too many confusions in this thread. Sorry about that.

CF

[Jon Marshall]

CF

No need to apologise, your involvement is always welcome.

To be honest I wasn't sure myself until I tested it and am making an assumption about why it is not advertising a route from the same major network but not with the same mask.

I just can't think of any other reason why it wouldn't be advertised at the moment.

Jon

[Jon Marshall]

It's a good question.

To be honest I had to lab it up to test.

I knew a, c and d already but wasn't sure about b. What happens is that this route is not advertised to router B.

I think it is because that network is also part of the same major network that is used for the link between the routers and so it cannot be advertised.

I should say I am testing on a simulator not real routers and CFs result suggest something else ie. a route of the same major network is advertised across the link.

I can't do a packet capture so I am just debugging RIP to see what is being advertised.

Jon