SIP ALG or SIP Application Layer Gateway is designed to facilitate the flow of VoIP traffic through your router, but often it creates more problems than it solves.  Many vendors ship their routers with SIP ALG enabled by default. We highly recommend that you disable SIP ALG on your router. This article includes links to further instructions for disabling SIP ALG with commonly configured routers.

What is SIP ALG?

Breaking this down: 

  • SIP is the industry standard protocol for VoIP
  • ALG is a common feature of routers designed to assist in managing certain types of internet traffic (in this case SIP traffic).

Why Disable SIP ALG?

By design, SIP ALG places itself in the middle of the communication path between your phones and Kixie.  This means that all communications and instructions relating to your VoIP service will be filtered through ALG.

Many different router vendors have their own varied implementations of SIP ALG, each of which has it's on quirks and limitations.  While most won't cause any noticeable problems most of the time, over time they can start to cause problems.

Common problems associated with SIP ALG include:

  • Phones ringing, but no audio when the call is connected (either one-way audio or no audio)
  • Sporadic issues with phones not ringing
  • Random phones losing their connection / not registering to the PBX
  • General Choppiness on calls

Kixie is designed to operate in such a way that SIP ALG is not required, so in almost all cases it is better to have it off than leave it on.

How to Disable SIP ALG:

The process for disabling SIP ALG varies depending on the router.  Please see the guides below for common routers.