In this example I share with you the logic and simple tricks with RIP version1.
Look at the picture provided:
In this picture we have taken two Cisco routers named R1 and R2. Both connected by LAN serial link, clock 64k, network 20.20.20.0 /24. Router R1 connects to two ethernet LAN segments with subnets 10.10.10.0 /24 and 10.50.50.0 /24. Router R2 connects ethernet LAN segment at 10.20.20.0 /24.When router one is up it has its routing table with 10.x.x.0 and 20.x.x.0 in its routing table showing directly connected. But since R1 knows it has two 10.x range subnets hence it has wrapped it under classfull boundary of class A. Any routing table gives highest priority to what it has on its interfaces.
Similarly, R2 knows 20.x serial range with 20.20.20.2 IP, s0/0 has not wrapped 20.x range to classful boundary since it has only one of its kind as on R1.
If suppose we were to have one more 20.x range like 20.30.30.0 it would have been wrapped to 20.0.0.0 /8 and then represented variably subnetted.
Well lets get to the point. Running RIP Version 1 which hardly forms neighbor and broadcast to every member interface by default. Even if we have not set RIP v1 on R2 , R1 will send update to all its connecting interfaces like f0/0, f0/1 and s0/0. point to be considered here is no one wants update to be sent on fast ethernet segments as we don't have rip neighbors there.
There is one more thing to be noted when we put network subcommand under router rip that we indirectly tell the RIP to send updates on those networks than to advertise the networks. So suppose if you have written
Router rip
ver1
net 10.10.10.0
net 10.20.20.0
net 20.20.20.0
The rip will summarizes them to class full boundaries of class A and advertise networks like 10.0.0.0 to interfaces members of RIP other than owner interfaces.
Now the logical mystery is that R2 though will receive the update via 20.20.20.1 s0/0 for 10.0.0.0 but it will not put in its routing table unless the 10.0.0.0 /24 entry for f0/0 is put down or disallowed.
When we put f0/0 down on the R2, it will learn 10.0.0.0 update from R1 and immediately will put it in routing table of R2 like this
When f0/0 on R2 was put down:
Gateway of last resort is not set
20.0.0.0/24 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Serial0/0
R 10.0.0.0/8 [120/1] via 20.20.20.1, 00:01:59, Serial0/0
Now if we put F0/0 of R2 up the entry will be something like this
Gateway of last resort is not set
20.0.0.0/24 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Serial0/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.20.20.0/24 is directly connected, Serial0/1
R 10.0.0.0/8 [120/1] via 20.20.20.1, 00:00:29, Serial0/0
Router(config-if)#
Now if there is a request for 10.x.x.x network it will be matched with highest bits and priority route will be directly connected f0/0 otherwise the 10.x.x.x via RIP on R1 will become the chosen route.
Now if there is a request for 10.x.x.x network it will be matched with highest bits and priority route will be directly connected f0/0 otherwise the 10.x.x.x via RIP on R1 will become the chosen route.
