basic ping question

[thomasmphil123]

hello all,i have an intresting question wot ma tutor asked me...

the queston is I hav 2 routers 1 AND 2 .

router 1 interface e0 and e1.router 2,interface e0 and e1.router1 e0 ip address is 192.168.3.1/24 and e1 ip address is 192.168.4.2/24.router 2 ip address for e0 is 192.168.4.1/24 and e1 ip is 192.168.3.2/24.Both the routers are directly connected.No routing protocols are used.The question was if i ping from router 1 to 4.1/24 will i get a ping ...justify your answer with explanation.can anyone tel me a solution for this.

[dhananjoy chowdhury]

When you ping 192.168.4.1 from Router1, it will go out through the int e1, as Router will prefer the directly connected interface.

Now if you have connected e1 of R1 to e1 of R2 back2back , then it should not get ping reply.

But again , all the four interfaces of R1 and R2 are connected in the same switch/VLAN , then you will get ping reply.

[Giuseppe Larosa]

Hello thomas,

the answer is yes.

all what is involved here is the ARP protocol to resolve 192.168.4.1 in r2:e0's MAC address you don't need a routing protocol for this.

But be aware that you need to have R1:e1 connected to R2:e0 with a cross-over RJ45 cable.

In your case if you connect r1:e1 to r2:e1 you still can ping r1:e1 from R1 but you cannot ping the other side.

How to detect the error in cabling ?

if you are not able to see an ARP entry for the remote router you can guess cables have been swapped.

Actually on LAN interfaces the icmp packet is not sent out on the link when it is directed to interface's ip address, but it is processed by the interface itself.

I used a trick when doing tests some years ago: or I had connected two lan interfaces with a cross-over cable, or I had disabled keepalive in a ethernet port.

In both cases I was able to ping the ethernet's port ip address from a remote router.

On WAN interfaces like ATM, FR the packet is actually sent out of the interface and the remote end will send it back to the router.

Hope to help

Giuseppe