[Asterisk-Users] best MTU?

[Leo Ann Boon]

Dr. Michael J. Chudobiak wrote:

> Hi all,
>
> 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?

Actually the maximum MTU varies from ISP to ISP. Usually, they will 
advise you when you subscribe. It's not only Sonicwall. In my previous 
job, we'd the same problem with a Cisco Soho DSL router. It can't do web 
with the standard MTU of 1500. The ISP advised us to change to 1476. I 
believe it's caused by the addition of PPPoE header to the frame. A 
standard Ethernet frame is 1500bytes. Adding the header to a full frame 
will cause the final frame size to exceed 1500, which causes fragmentation.