[Asterisk-Users] best MTU?

[Rich Adamson]

> I have several locations, each connected by a Sonicwall VPN through 
> PPPOE DSL, with Snom 360 phones.
> 
> I've found that I have to tweak the Asterisk server MTU (inside one of 
> the firewalls) to get everything to work "just right". Set the server 
> MTU too low, and the Snom phones don't communicate correctly anymore. 
> Too high and the phones work, but the server can't access the web (for 
> yum updates, for example).
> 
> So I've settled on an MTU of 1448.
> 
> Has anyone else struggled with this? What did you settle on?

I've not experienced the problem with asterisk (but that's because I've 
not implemented anything as yet behind a Sonicwall).

Sonicwall does have a problem with the mtu when using pppoe, and that 
problem has been around for several software releases. Not sure why they 
can't get it right, but they can't.

The limited testing that we've done suggests the mtu that you've picked 
is reasonable. However, you'll still run into a problem anytime the user 
on the LAN side of the firewall hits a box on the outside that happens 
to be sending pkts based on a standard ethernet mtu. The only way to 
correct "that" is to change the mtu at the distant location. (Obviously, 
if that device is not under your control, it can't be fixed. It seems to 
only be an issue with large udp packets. Not seen the problem with tcp 
pkts.)

We've sent detailed packet traces to the Sonicwall engineers 
specifically showing the problem, but that was about two years ago and 
the problem is still around.