Identifying SIP ALG

SIP ALG (Application Layer Gateway) is a security component of most commercial routers. In addition to router firewalls, SIP ALG modifies fax and audio data packets, helping the Local Area Network (LAN) determine which packets to allow while also keeping hazardous data files at bay.

 

 

SIP ALG solves issues related to NAT (Network Address Translation) on older commercial router models. Both NAT and SIP ALG security components can coexist in the same router, sometimes creating mixed signals in how or if data packets from the public telephony can reach local devices.

Although NAT and SIP-ALG share the same purpose, they do not necessarily speak the same language. If SIP ALG is on, which is the default on most routers, some of the voice files will not reach the office devices. This will result in missed calls, broken lines, and sudden service interruption.

Most routers only affect SIP ALG messages sent on port 5060. For this reason, Nextiva signals use port 5062 for registration.

NOTE: Some routers will allow SIP ALG messages to be displayed, but for those that do not, using port 5062 for registration is a must.

Symptoms of SIP ALG Include:

  • One-way audio on calls
  • No audio
  • Devices dropping registration
  • Calls going straight to voicemail for no known reason
  • Random error messages for incoming calls (e.g. “The caller you have dialed is no longer in service.”)

If SIP ALG causes issues, Users can disable it on most routers. Otherwise, Users must register the device on port 5062.

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Updated on November 14, 2019

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