question about classful routing

nleslie1970

Currently studying for the ccna and there is a little bit of confusion over classful routing.

Let's say we have 2 routers each 2 interfaces, a fastethernet0/0 and a serial0/0. And we are using routing protocol RIP v1. Serial interfaces are plugged into one another as well as the ethernet ports.

Router1 s0/0 - 10.1.1.1/24
Router1 fastethernet/0/ - 10.1.2.1/24

Router2 s0/0 - 10.1.1.2/24
Router2 fastethernet0/0 - 10.1.2.2/24

Since they are all on the same subnet, will RIP know that these are 2 separate networks or will it just broadcast 10.0.0.8/8 and think of this as one network?

I'm a little confused because the book i'm using (todd Lamme) has an example similar to this except i think he uses 4 routers. But he has everything on the same subnet (/24) but is using class A networks. He has RIP routing the various networks, 10.1.1.0, 10.1.2.0,10.1.3.0 and so forth.

I guess my initial thought would be that classful routing would mean RIP would send the updates as the classful boundary (10.0.0.0/8 and routing wouldn't work but i assume it does if they are on the same subnet?

any light that can be shed on my confusion would be much appreciated.

sizeon

That is not going to work. You can't have host with same subnet on different network. You have fastether on the same subnet on both routers.

lantech

How did you get that you have only 1 subnet and a summarized route of 10.0.0.0/8?

You have two subnets.

Subnet 1 is 10.1.1.0/24 which is on your serial interfaces
Subnet 2 is 10.1.2.0/24 which is on your fast ethernet interfaces.

Are your fast ethernet interfaces connected to each other or are they connected to another device such as a switch or host device. If they are connected to each other then they should work. If they are connected to a switch or host device then it won't work because you have one subnet spanning two routers. With that setup the routers won't know which interface to send the packets out of.

NetworkVeteran

OP wrote:

Serial interfaces are plugged into one another as well as the ethernet ports
Router1 fastethernet/0/ - 10.1.2.1/24
Router2 fastethernet0/0 - 10.1.2.2/24

sizeon wrote: »

That is not going to work. You can't have host with same subnet on different network. You have fastether on the same subnet on both routers.

Per the OP's description, the FastEthernet interfaces are connected to each other, so the two devices with the same subnet are on the same LAN. This configuration will work just fine.

OP wrote:

I guess my initial thought would be that classful routing would mean RIP would send the updates as the classful boundary (10.0.0.0/8 and routing wouldn't work but i assume it does if they are on the same subnet?

RIP will send updates for 10.1.1.0 and 10.1.2.0 in this scenario, but not for 10.0.0.0.

Classful vs. Classless is not Subnets vs. No Subnets. The classful assumption is that the class A address 1.0.0.0/8 is assigned to an organization. That organization may then subdivide (subnet) 1.0.0.0/8 into 1.1.0.0/16 and 1.2.0.0/16. In classless routing, 1.1.0.0/16 and 1.2.0.0/16 could be assigned to different, unrelated organizations. In networking parlay, discontiguous.

More specifically, both RIPv1 and RIPv2 would both handle this situation fine and advertise the subnets. RIPv1 would send 10.1.1.0 and 10.1.2.0. RIPv2 would send 10.1.1.0/24 and 10.1.2.0/24.