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.

Solution:

i) Check the firmware version on your router. 3.3.4.1_232201 (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.