Question about VLSM and RIP

Hello,
I've found this wonderful website and maybe I am asking too many question in too short period of time. I apologize for that.

Well, here it is my third question in two days:

I have a Router R1 with 2 Ethernet cards E0 and E1 connected to a second router R2 through S0. Second Router R2 has also two Ethernet ports E0, E1 and a serial interface (connected to R1).

R1
!
int e 0
ip address 172.12.10.0 255.255.255.0
!
int e 1
ip address 172.12.20.0 255.255.255.0
!
int s 0
ip address 172.12.30.0 255.255.255.252
!
router rip
network 172.12.0.0

R2
!
int e 0
ip address 172.12.40.0 255.255.255.0
!
int e 1
ip address 172.12.50.0 255.255.255.0
!
int s 0
ip address 172.12.30.0 255.255.255.252
!
router rip
network 172.12.0.0
!

The book says that the routing table of r1 will show knowledge of networks 172.12.40.0 and 172.12.50.0 as learned through RIP and the routing table of r2 will show knowledge of networks 172.12.10.0 and 172.12.20.0 also learned through RIP.
My question gets here:
How the routers learned about subnets 10, 20, 40 and 50 ?
It is not supposed that 10,20,40, and 50 get lost when used the classful address of class B (172.12.x.x) ?
Or the VLSM means that RIP can not adverstized different subnet marks ( and in this case S0 it is using a 30-bit subnet versus 24-bit subnets of the rest of the interfaces ?
So, If this it's the case, I can interpret Variable as Different ?

I am not sure If I ask a clear question, but I will be very happy to explain my concern in a different way, if asked to.

Thanks a lot for reading as far,
Regards,
Andres

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Comments

pitusa

Now in Cisco website, I found that the only way the network will be advertized is if the interface advertizing has the same subnet than the subnet on the other interfaces part of the network advertized. Kind of confirming my understanding that it is not the subnetting but the existance of different subnet masks on the router interfaces.
Am I right ?

Thanks

pitusa

So, coming back to the example I placed on my first post on this thread. None of the subnet will be adverstized to the other router because the advertising interfaces (S 0 in this case) has a different lenght subnet mask than the networks on the Ethernet interfaces.
Am I still on track ? or I lose it 100% ?

Thanks for any comment.

Sartan

Hmmz...
Do a websearch for "Route Summarization"
As well, remember that Ripv1 will not advertise network masks. V2 will.

Webmaster

I'm still not entirely sure what your main question is, and there are some things wrong inyour config example, i.e.
ip address 172.12.10.0 255.255.255.0 is not possible... it denotes an entire network.

RIP defaults to version 1 (sends only version1 packets, listens to both version 1 and 2), which does not support variable subnet masks. When the ip addresses are changed to valid ones, and the command 'version 2' is added below router rip (in router config mode), the configuration will work just fine. If you don't set it to version 2 it won't even learn the routes thru RIP, and the routing table will only list (sh ip ro) the directly connected subnets.

When you use the network command to add networks, you actually tell the router which interfaces should be added to the routing process. RIP will learn the subnet masks from the interface's configuration. Even RIP version 1 will do that, which you can confirm by using RIP version 1 and checking the routing table. It will list the network '172.12.0.0/16 is variably subnetted'. Version 1 just doesn't has a field for the subnet mask in RIP update packets, but a RIP version 1 router does know about the subnet mask, also if they are variable, they just cannot share that knowledge with other routers.

There is no summarization in this example. Summarization occurs at the network boundary, i.e. where 192.168.1.0 meets 172.12.0.0

And when it comes to Cisco exams there is never need to do a web search... do www.cisco.com search instead

I've found this wonderful website and maybe I am asking too many question in too short period of time. I apologize for that.

That's what the forums are for, ask as much as you need.

pitusa

Thanks for the answers,
Yes, I typo the ip addresses of the interfaces. I meant that they belong to those networks (I was thinking about the network and not the particular interface...I'm hoping for typing better in my exam

Back to my question:

If Rip v1 do not send any information about the subnet mask, how the neighbor router R2 learn about 172.12.10.x and 172.12.20.x (as shown in the sh ip route on R2) ?
I understood that no subnet is adverstized, so what R2 will do when receive a packet intended to 172.12.10.0 for example ?

Thanks a lot for your pointers,

Sartan

pitusa wrote:

Thanks for the answers,
about the subnet mask, how the neighbor router R2 learn about 172.12.10.x and 172.12.20.x (as shown in the sh ip route on R2) ?
I understood that no subnet is adverstized, so what R2 will do when receive a packet intended to 172.12.10.0 for example ?

RipV1 will advertise 172.12.0.0 through all of it's interfaces.

00000000 ~ 0 - Class A
10000000 ~ 128 - Class B
11000000 ~ 192 - Class C

Webmaster

pitusa wrote:

If Rip v1 do not send any information about the subnet mask, how the neighbor router R2 learn about 172.12.10.x and 172.12.20.x (as shown in the sh ip route on R2) ?

R2 won't learn about these subnets.

I understood that no subnet is adverstized, so what R2 will do when receive a packet intended to 172.12.10.0 for example ?

This depends on the routing table and if the ip classless command is being used. But in your example, it won't have a route for it. The only routes in the routing tables based on your example config (assuming version 1) will be those that are directly connected (none learned thru rip). Again, the local router knows about the subnets, it will learn those from its own interfaces. show ip route output on R2 (with RIP version 1) will be this:

172.12.0.0/16 is variably subnetted, 3 subnets, 2 masks
C       172.12.40.0/24 is directly connected, Ethernet0
C       172.12.50.0/24 is directly connected, Ethernet1
C       172.12.30.0/30 is directly connected, Serial0

As you can see there is no route to 172.12.10.0

pitusa

Perfectly clear now.
Thanks!!!

Webmaster

You're welcome.