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P-2302HWUDL-P1 Series User’s Guide
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APPENDIX E
SIP Passthrough
Enabling/Disabling the SIP ALG
You can turn off the ZyXEL Device SIP ALG to avoid retranslating the IP address of an
existing SIP device that is using STUN. If you want to use STUN with a SIP client device (a
SIP phone or IP phone for example) behind the ZyXEL Device, use the ip alg disable
ALG_SIP command to turn off the SIP ALG.
Signaling Session Timeout
Most SIP clients have an “expire” mechanism indicating the lifetime of signaling sessions.
The SIP UA sends registration packets to the SIP server periodically and keeps the session
alive in the ZyXEL Device.
If the SIP client does not have this mechanism and makes no call during the ZyXEL Device
SIP timeout default (60 minutes), the ZyXEL Device SIP ALG drops any incoming calls after
the timeout period. You can use the ip alg siptimeout command to change the timeout
value.
Audio Session Timeout
If no voice packets go through the SIP ALG before the timeout period default (5 minutes)
expires, the SIP ALG does not drop the call but blocks all voice traffic and deletes the audio
session. You cannot hear anything and you will need to make a new call to continue your
conversation.