When discussing VoIP problems with our support team we may ask you whether SIP ALG is enabled on your router. This article intends to explain what it is, how it can affect VoIP and why we recommend you turn it off.
What is SIP ALG?
SIP ALG stands for Application Layer Gateway, and is common in many commercial routers. It intends to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it.
Many routers have SIP ALG turned on by default.
How can it affect VoIP?
Even though SIP ALG is intended to assist users who have phones on private IP addresses, in many cases it is implemented poorly and actually causes more problems than it solves. Gradwell and other providers often find that the SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behaviour, such as phones not registering and incoming calls failing.
Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled.
How do I turn off SIP ALG?
The method to disable SIP ALG will depend on your router. In the first instance, we recommend checking the manufacturer’s handbook that came with your device. Alternatively check the internet and support forums for advice (type your router make and model into a search engine, along with “disable SIP ALG”).
Please note that if you have a router from Gradwell for our DSL service then SIP ALG is turned off by default.
I have disabled SIP ALG but I’m still experiencing problems
If you are still having problems after disabling SIP ALG, please check your firewall configuration.