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Testing VoIP connectivity on IOS

An undocumented command that makes testing easy.


Testing VoIP connectivity on IOS
Tom Lancaster

Occasionally, you may be placed in a situation where you need to configure a Cisco IOS router, which has FXS ports, so that someone can plug a phone into one of the ports and dial a number, which will in turn cause the router to set up a VOIP call to some destination. If you're like most people, you'll want to test this to make sure it works, especially since VOIP technology is new enough that many people aren't comfortable with it yet. However, testing it can be a challenge since you probably don't carry a phone around with you, or there may be a phone plugged to the router but you may not be physically at the router, so you can't simply pick up that phone and dial a number.

One possible way to test the connectivity to make sure your configuration is correct is to use the undocumented IOS command:

csim start <phone#>

This command is issued from the privileged exec mode and when it's successful, it generates a message similar to this, where 5551212 is the number dialed:

router#csim start 5551212
csim: called number = 5551212, loop count = 1 ping count = 0
csim err csimDisconnected recvd DISC cid(21)
csim: loop = 1, failed = 1
csim: call attempted = 1, setup failed = 1, tone failed = 0

That may not look like a successful call, but it is. When the call is NOT successful, you will see a message like this:

csim: called number = 5551212, loop count = 1 ping count = 0

csim err:csim_do_test Error peer not found

While this command can really help you out, be advised that it has been known to cause some problems. After all, it's undocumented for a reason. As with all undocumented commands, use at your own risk!

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

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