How to Disable a SIP ALG

An ALG is an Application Layer Gateway, which is supposed to help alleviate issues with certain protocols. Unfortunately, most ALGs tend to introduce more problems than anything else. This is very true for SIP ALGs! When a poorly engineered SIP ALG is enabled it often causes the following problems:

Signaling Issues

Signaling problems occurs when the SIP protocol is unable to communicate between two endpoints, or with an endpoint and a SIP Proxy. A SIP proxy could be a Session Border Controller or a PBX. When you have a signaling issue, you may have a lot of missed calls, or you will dial a number and the request will timeout.

Audio Issues

In my experience, the most common issues for SIP ALGs involves the audio streams. “One Way” audio is at the top of the list. When you experience one-way audio, one person is able to speak and hear the other party, but the other person is unable to receive the audio coming back to them. This leads to frustration and people hanging up the calls.