RIPv1 subnet mask question

[dannan lin]

hi:

thanks for taking time to answer my question.

in following situation

Router 1 :

f0/0: 192.168.1.1 /28             255.255.255.240

lookback99: 192.168.1.9/29   255.255.255.248

Router 2:

f0/0: 192.168.1.2 255.255.255.240  

both running rip v1

I know RIPv1 does not send netmasks , it only assumes that the incoming net address has the same mask as its receiving port.

for example if 192.168.1.2/28 was the receing port  address, it will assumes all incoming net address mask is  255.255.255.240 right ? 

is there any experiement i can do to prove this?

Will Router 2 consider loopback interface from Router 1 as /28 mask?

and also

i also realised i cannot set loopback mask lower than /28, if i do that i will get message saying address overlap with f0/0

why is this ?

thanks in advance.

[Richard Burts]

The mask that you can use for the loopback interface can not be lower than /28 when you are assigning .9 as the interface address. If you think about it (or if you consult a subnet calculator) if the address is .9 and the mask were /27 then the .9 address is in a subnet that begins at 1 and goes up through 15 - which does overlap with the subnet of f0/0. If you were to use a different address you might be able to use a different mask. For example this should work just fine:

inteface loopback0

ip address 192.168.1.129 255.255.255.128

The situation that you describe is a fairly good simple experiment with 2 interfaces with different masks. If you do set this up on a pair of routers then you would discover an interesting thing about RIPv1. Router2 will not learn the loopback interface address because Router1 will not advertise it. RIPv1 will not advertise out f0/0 a subnet whose mask is different from the mask of f0/0.

HTH

Rick

[dannan lin]

thanks .