Cannot ping local serial interface

[mario.milhazes]

Hi

My name is Mário and i´m a CCNA and CCAI

When i was teaching a class a problem ocurred while configuring two routers with a back-to-back V35 cable.

The two serial interfaces that conected the routers were configured with IP address from diferent networks, until here every thing is ok the routers could not reach each other.

What is odd is that i could not ping my local serial interface, i could only ping the local interface if i shut down the neighbors serial interface.

Can anyone please explain why does this happen.

Thanks Mário

[Kevin Dorrell · Accepted solution]

When you ping your own address on a point-to-point interface, it actually sends the ping to the other router, and the other router sends it back. You then generate a response to the ping and send it to the partner, which then sends it back. So a ping to your local serial address actually traverses the line twice, and the response also traverses the line twice.

If the partner is set up to a different subnet, it will not have a route back to you through the serial line. That is why the ping failed.

Now .... why did it work when you shut down the neighbor's serial interface? I have not tried this myself, but here is something to investigate: Normally, when you shut down the neighbor's interface, your interface will be marked down, so the ping should fail. So I am guessing that you were using the default HDLC encapsulation, and you had configured no keepalive to force your end of the link up. The question is, what happens to the ping when the local interface is spoofing "up" in this way. To be honest, I don't know off the top of my head, but I would be interested to try it out.

Kevin Dorrell

Luxembourg

[mario.milhazes]

Thanks Kevin, you have been very helpfull

[Richard Burts · Accepted solution]

Mario

The explanation is that when you ping your own serial interface, the router actually transmits the ping request over the wire. Normally the ping request is received by your neighbor on the other end of the point to point connection and the router recognizes that it needs to forward the packet to the destination and forwards it back over the serial connection since it is the connected subnet. In your case the ping gets to the neighbor, the neighbor looks at the packet, wants to forward to the destination but since the destination is no longer a connected subnet it does not know how to forward the packet.

Why are the routers configured with different network addresses?

If you want to keep them as different addresses and want to be able to ping your own interface, then you will need to configure a static route on each router pointing to the subnet of the neighbor serial interface.

HTH

Rick

[mario.milhazes]

Thanks Rick

The IP addresses were configured in diferent networks because we were doing a IP address troubleshooting Lab, that told us to put IP addresses from diferent networks in a point-to-point connection and to se what happend.

[Richard Burts]

Mario

So you saw what happened. That would make an interresting troubleshooting lab.

HTH

Rick