pinging looped-back serial interfaces

[asmolecki]

This has been an issue with several of our technical folks and I'm hoping to get this cleared up somehow. The issue concerns troubleshooting a serial interface (T1, for example) and the circuit attached to it. Some believe that when the interface is in an up & up (looped) status with the far end of the circuit being looped-up that pinging the interface from within the same router will send frames through the looped circuit and back to router and qualifying as a valid test of the circuit and interface. Others believe that the pinging does not send frames through the circuit and only tests the interface. I suppose I could throw a sniffer in-line with the circuit to find this out, but I'm hoping someone might know the answer to this already.

Any reply would be much appreciated.

Art

[n2-goes]

Hello Art,

It depends if you put the interface in loop than the ping will enter the interface and exit the interface. You can also put the loop on the csu\dsu in that case the ping will go up to the csu\dsu and come back. If you ask the telco to put the loop on the far end of the circuit the ping will travel all the way to the far end. And finally you could loop the other router interface.

This way you could test the circuit end-to-end.

HTH,

Nadine.

[Richard Burts]

Cisco routers on a serial interface do send packets on the wire. So if you have the remote end of the circuit looped and issue a ping, the ping packets will traverse the circuit to the loopback and then return.

[prafuljaded]

You can configure " down-when-looped" command on the serial interface so that the circuit shows line up, protocol down( looped) during loop testing.Ping doesn't work for this command.

[singhshekhawatv1]

hi,

My MPLS ckt is flapping i need to know that when ckt will be looped towards MPLS. than it can be ping or not at peer end??