Soft loopback

Troubleshooting serial link through soft Loopback testing on CISCO Routers
http://technicafe.net/2012/05/troubleshooting-serial-link-through.html

Note: Softloop test should only be performed when serial link is down. If performed when the link is up, it will effect the data transmission.

Troubleshooting serial links is always critical issue in networking world. One of the smarter way to test the connectivity through different hops by looping, which can either be software based or hardware based.

Soft looping:
Soft looping is a method which feeds a required signal or data, back to the sender. Through soft looping physical connection problems, passing through various Hops can be rectified. In Cisco routers, soft loopback is implemented with a Cisco IOS configuration command, or with a loopback button on some CSU/DSU.
(Channel Service Unit/Data Service Unit)

While working on a leased line or dedicated line, provided by ISP, there are times when we know there is some issue in leased line, then we can perform loopback test through our Cisco IOS and can do some measurement to rectify the exact problem.
In Cisco IOS, following soft loopback modes can be applied while testing :
Local : Loops the router data towards router and sends AIS towards the network.

Syntax :
loopback [remote {line {fdl ansi | inband} | payload fdl ansi}]

Link status can be analyzed through show interface serial x/x/x:x command in Cisco IOS. Point to remember is that whenever an interface will be looped, the output of show interface command will always be showing serial x is up, line protocol is down(looped).