Draytek 2820 Routers and SIP Handsets

This is a quick post for anyone who is having Draytek Vigor Routers + SIP problems. It’s taken me weeks to get to the bottom of these issues, so hopefully it’ll save others the time. It’s worth mentioning that I have only had issues and tested routers with the UK firmware and have no idea if these issues exist in other version.

Symptoms: Registrations from SIP handsets behind 2820 routers in NAT mode randomly becoming unavailable and refuse to re-register. This exhibits itself on the SNOM 300 handsets that we predominantly use with the dreaded ‘Network Failure’ message.


i) Check the firmware version on your router. (the lastest at the time of writing) seems to have some serious issues with SIP handsets. I have reverted back to 3.3.3_232201 which seems stable so far (fingers crossed)

ii) Make sure ‘Accept large incoming fragmented UDP or ICMP packets’ is disabled. This can be done in the web interface under: Firewall=>General Setup by unticking the checkbox

iii) Disable SIP_ALG. You’ll have to telnet into your router to do this. Once logged in you will have to issue the commands:

1. > sys sip_alg 0 — Disables sip alg

2. > sys commit — Apply changes

3. > sys reboot — Reboot router

I hope this is of use to someone.