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 causes more problems than it solves. Gamma 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, One Way Audio/Media issues and calls failing to transfer properly. SIP ALG must be turned off for Horizon to work off and the service is not supported until SIP ALG is turned off.
How can I check if SIP ALG is affecting my customers’ service? #
You can check if SIP ALG is affecting your customer’s service by using the Health Check.
How to switch off SIP ALG using Telnet #
This guide is going to tell you how to use Telnet (a text-based protocol that allows you to connect to a remote device over the Internet) through command prompt, though there are programs such as SecureCRT that you can use for this.
Gamma offer no further support for using Telnet to switch off SIP ALG other than these guides. Gamma takes no responsibility for anything that may happen to your router. It may also invalidate your router’s warranty so you may want to check this before proceeding. You should only proceed if you are completely happy to do so. You should always speak to your Internet Service Provider or consult your router user guide for further information.
Before you start you should see the Microsoft Help Pages to ensure that you have Telnet enabled in Windows first.
Cisco Routers #
Step 1
On your PC go to “Start” and select Run. Type in “cmd” to load up the Command Prompt.
Type into the following to Command Prompt:
“Telnet” followed by the IP address of your router
If you are doing this on the LAN then this will be your Default Gateway in “IPConfig”. If you are doing this on an external network then you will need to ask your customer to go to Google and what in “What’s My IP”. More information can be found at About.com.
Step 2
Type in “enable” and then enter your administrator’s login when asked for “Account” and then the password is the password of the router.
Step 3
Type in “Configure terminal”.
Step 4
Type in the following commands:
No ip nat service sip tcp port 5060
No ip nat service sip tcp port 5060
Step 5
Exit out of the configuration terminal by pressing “Ctrl+C” together.
Type in “Copy run start” to save the configuration.
Step 6
Type in “Reload” and hit enter which will reboot the router.
Draytek Routers #
Step 1
On your PC go to “Start” and select Run. Type in “cmd” to load up the Command Prompt.
Type into the following to Command Prompt:
“Telnet” followed by the IP address of your router
If you are doing this on the LAN then this will be your Default Gateway in “IPConfig”. If you are doing this on an external network then you will need to ask your customer to go to Google and what in “What’s My IP”. More information can be found at About.com.
Step 2
Type in “enable” and then enter your administrator’s login when asked for “Account” and then the password is the password of the router.
Step 3
Type in the following commands and hit enter:
sys sip_alg 0
Step 4
Type in the following commands and hit enter:
Sys commit
Step 5
Type in the following commands and hit enter:
Sys reboot
Step 6
Once your router has rebooted you’ll need to go into the Firewall settings on your router through the router’s GUI and make sure that you “Accept large incoming fragmented UDP or ICMP packets” or something similar is not selected.
Reboot your router again and you’re good to go.
Technicolour Routers #
Step 1
On your PC go to “Start” and select Run. Type in “cmd” to load up the Command Prompt.
Type into the following to Command Prompt:
“Telnet” followed by the IP address of your router
If you are doing this on the LAN then this will be your Default Gateway in “IPConfig”. If you are doing this on an external network then you will need to ask your customer to go to Google and what in “What’s My IP”. More information can be found at About.com.
Step 2
Type in “enable” and then enter your administrator’s login when asked for “Account” and then the password is the password of the router.
Step 3
Type in the following commands and hit enter:
Connection unbind application=SIP port=5060
saveall
Zyxel Routers #
Step 1
On your PC go to “Start” and select Run. Type in “cmd” to load up the Command Prompt.
Type into the following to Command Prompt:
“Telnet” followed by the IP address of your router
If you are doing this on the LAN then this will be your Default Gateway in “IPConfig”. If you are doing this on an external network then you will need to ask your customer to go to Google and what in “What’s My IP”. More information can be found at About.com.
Step 2
Type in “enable” and then enter your administrator’s login when asked for “Account” and then the password is the password of the router.
Step 3
Go to Menu option “24. System Maintenance” Then select Menu option “8. Command Interpreter Mode”. Type in the following command and hit enter:
Ip nat service sip active 0
Step 4
Type in the following commands and hit enter:
Sys commit
Step 5
Type in the following commands and hit enter:
Sys reboot
Switch off SIP ALG on a Netgear Router #
Gamma offer no further support to switch off SIP ALG other than these guides. Gamma takes no responsibility for anything that may happen to your router. You should only proceed if you are completely happy to do so. You should always speak to your Internet Service Provider or consult your router user guide for further information.
Step 1
In your web browser type in “198.168.0.1” and press enter. This will then load up your router’s configuration GUI.
Use the following details to log in:
Username: admin
Password: password
If you have confirmed a custom password previously then you should use this instead of “Password”. If these combinations don’t work you should consult the Netgear router user manual.
Step 2
Select “Advanced” and then “WAN Setup”
On some version of Netgear firmware you may need to select “NAT Filtering”.
Step 3
Click on the “Disable SIP ALG” check box and then click “Apply”.