Err ma gerd, SIP ALG

Seriously, I don’t know what has changed, but this SIP ALG is the bane of Skype/Lync existence, and if it isn’t the corporate firewall, then it’s the home based users firewall.  Others have blogged on this, so mostly I’m just spreading the word.

It used to be that MS Skype for Business and Lync were so encrypted, that SIP ALG couldn’t touch it, so ALG settings didn’t affect it.  Un-encrypted SIP traffic such as Internet Telephony Services Providers (ITSP’s) SIP Trunks with Intelepeer, ThinkTel, etc, you especially had to make sure SIP ALG was disabled.

Main issue that is showing up, is that Desktop Sharing for home-based Skype for Business users.  Audio quality may also be impacted as well.

AT&T U-Verse:

  • Under Firewall, disable  Flood Limit and SIP ALG


  • Under Firewall | IPv4, set Firewall Security Level to Medium

Telus Routers:

  • Under Advanced Setup | Modem Utilities

Netgear, D-Link, SonicWall, AT&T 2WIRE:

I do not have instructions for other internet providers at this time, but if you do, I’ll add them to the list if you post them in the comments section.

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