[Asterisk-Users] best MTU?

[James Harper]

> 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?

Can you give a better description of your network? (ascii art is
acceptable :)

Just out of curiosity, what sort of MTU values were you finding 'too
low', and on what interface were you setting it (I assume pppoe as you
mention it above)?

You can find that vpn tunnels break pmtu discovery (you are sending
packets that are too big, but the 'packet too big' message gets blocked
or lost so you never find out). Can you do a 'tracepath' to the remote
phone? That will determine the mtu and tell you at each hop, and can
show where packets are being lost.

James