Soft Loopback

[Nixon]

Good day all,

I just want to know if the soft loopback will work on PPP encapsulated serial interface.

I tried it on HDLC encapsulated serial interface and it woks fine. However not on PPP encapsulated serial interface.

Thanks. More power.

[Giuseppe Larosa]

Hello @Nixon ,

it is not clear what you mean with soft loopback.

However, PPP encapsulation sends a field named magic number and so a sync serial interface with PPP encapsulation can detect a loopback on the path by the fact that it starts to receive frames with its own magic number and declares the circuit looped.

So in real world for packet over sonet interface or other slower serial interface changing the encapsulation from HDLC to PPP is a required step in troubleshooting to demonstrate that a far end loopback is working and the local device detects the loop.

Hope to help

Giuseppe

[Nixon]

Hi Giuseppe,

Thanks for the reply. Here’s Cisco’s definition of soft loopback.

Soft loopback testing is accomplished with a set of Cisco IOS® software configuration commands on the Cisco gateway. The commands cause the WAN interface card (WIC) driver to automatically send traffic back towards the sending T1/E1 port.

Soft loopback does not require any hardware changes or re-configuration

[Giuseppe Larosa]

Hello @Nixon ,

thanks for your kind remark and your feedback on the meaning of soft loopback.

Actually some types of serial linterfaces allow to create the loopback using a command instead of a special cabling.

This is useful in troubleshooting.

Some ATM or Frame Relay switch could be configured to loop towards network or towards user using configuration commands.

Best Regards

Giuseppe