vigor 2920 problems

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One of our clients has a Draytek Vigor 2920- their natted Snom phones
behind it are registered to an Asterisk 1.4 server on an external public IP.

I’ve set QOS, bandwidth management and turned off the SIP ALG via telnet
but I’m still having some problems with some of the phones losing
registration if Asterisk is restarted.

I can see the phones sending SIP REGISTER messages, but they never arrive
at the server; this happens in about half of the phones- with no
consistency as to which lose registration.

It looks like the router is swallowing the messages, or there’s some kind
of NAT problem. Other clients at other sites are fine.

The problem clears if the phone is rebooted (renegotiates a new nat path?)

Any help warmly appreciated.


3 thoughts on - vigor 2920 problems

  • Hi John,

    We’ve had similiar issues with customers behind the 2920 connecting to a hosted asterisk system. If you rebooted a phone it often didn’t re-register, Checking the NAT sessions table on the router revealed stale nat sessions open for the phone.

    On the advice of Dreytek we found a fix by lowering the NAT session timeout from the default of 24hrs down to 5 minutes and installing the latest release of the firmware (3.3.7) it may not be available on the UK Site at the moment (It wasn’t when we did the upgrade!) but it can be got from

    It may help, It may not – But its quick easy fix if it does.


  • Thanks AJ- have set it to 5 mins via telnet: srv dhcp leasetime 600. Will
    get permission to try new firmware later!