3

I didn't configure any routing protocol to any of the router and tried to apply static routes in order to get out of the network and I've configured static routing in a manner that if R1 sends a ping to R3, the ping should go through R2 but the reply from R3 should come from R4 but it's not working. WHY?

Here are the ip addresses and static ip addresses:

R1#show ip int br | i up
Serial0/0                  192.168.1.1     YES NVRAM  up                    up
Serial0/1                  192.168.4.1     YES NVRAM  up                    up
R1#show running-config |  sec ip route
ip route 192.168.2.2 255.255.255.255 192.168.1.2

R2#show ip int br | i up
Serial0/0                  192.168.1.2     YES NVRAM  up                    up
Serial0/1                  192.168.2.2     YES NVRAM  up                    up
R2#show running-config | sec ip route
ip route 192.168.3.1 255.255.255.255 192.168.2.1

R3#show running-config | sec ip route
ip route 192.168.4.2 255.255.255.255 192.168.3.2
R3#show ip int br | i up
Serial0/0                  192.168.3.1     YES NVRAM  up                    up
Serial0/1                  192.168.2.1     YES NVRAM  up                    up
R3#show running-config | sec ip route
ip route 192.168.4.2 255.255.255.255 192.168.3.2

R4#show ip int br | i up
Serial0/0                  192.168.3.2     YES NVRAM  up                    up
Serial0/1                  192.168.4.2     YES NVRAM  up                    up
R4#show running-config | sec ip route
ip route 192.168.1.1 255.255.255.255 192.168.4.1

scenario:

enter image description here

1

In princple, Ron has correctly indicated the reason for your lab not working, but I am afraid it will not work with proposed two corrections too. This one is OK:

R3#ip route 192.168.1.1 255.255.255.255 192.168.3.2

I should mention that you don't really need this one, if you don't plan to ping 192.168.4.2:

R3#ip route 192.168.4.2 255.255.255.255 192.168.3.2

Now to R4: here you had already

R4#ip route 192.168.1.1 255.255.255.255 192.168.4.1

so nothing to correct here.

What is still missing to make things work, is

R1#ip route 192.168.2.1 255.255.255.255 192.168.1.2

Again, you don't need

R1#ip route 192.168.2.2 255.255.255.255 192.168.1.2

but you can have it if you like

So, add two routes on R1 and R3 and you'll see some !!!!! :)

3

You don't have a route to the ping source address (192.168.1.1) on R3 which goes back through R4. Your route to R3 from R1 is only for the network between R4 and R1, not the network on which the ping originated.

You are not pinging and replying to routers; you are pinging and replying to addresses in the IP packets. Without a route back to the source address in the destination router, the reply will be lost.

R3#ip route 192.168.1.1 255.255.255.255 192.168.3.2

Also, R4 will need a route to the source address so that it knows what to do with the reply since the original source address will be the destination address of the reply.

R4#ip route 192.168.1.1 255.255.255.255 192.168.4.1
2
  • I didn't get your answer or may be you didn't get my question. All I wanted to do is to send a ping from R1's se0/0 interface to R3's se0/1 interface so the ping should go from R2->R3 and reply should come from R3->R4 but it's failing. WHY?: R1#ping 192.168.2.1 source serial 0/0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds: Packet sent with a source address of 192.168.1.1 ..... Success rate is 0 percent (0/5)
    – Heisenberg
    Jan 14 '16 at 9:21
  • It is failing because you don't have a route on R3 and R4 back to the source interface address. R3 has no path back to the address which sent the ping, so it cannot reply. You seem to be assuming that if R3 has a path back to R1, that should be enough, but that is wrong since you are not pinging and replying to routers; you are pinging and replying to addresses, and without a route to which R3 can send the reply, the reply gets dropped. Implement what I have given you, and it should work.
    – Ron Maupin
    Jan 14 '16 at 14:22

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