Hi I have just been looking at the Draytek page for setting MTU:

http://www.draytek.co.uk/support/kb_vigor_mtu.html

This is now out of date for the newest firmware as you can now do wan mtu xxxx and wan mtu2 xxxx

Is there any relevant documentation in place, as our download speed has dropped terribly and I am trying to tweak MTU properly. Incidentally Draytek people, the bottom of that page shows:

Page last modified 10 Oct 2105

thanks
Rob

It is out of date but even more so now since it can be changed from inside the web interface (for WAN1) from 3.3.4 firmware onwards

Nice spot - didn't even notice that before!

I've done some testing with this (Linux, "ping -M do -s <size> <ip address>"), and I'm not convinced that my 2820 takes any notice of the WAN1 MTU setting. My MTU is set to 1458; however, pinging outwards from my LAN doesn't fail until I get to 1473 bytes. I think this is 1473 + 8 bytes for the ICMP header + 20 bytes for the IP header - i.e 1501 - indicating that the MTU is actually 1500.

Also there seems to be some confusion as to what the optimum value should be. Before 21CN, it was 1458, whereas for 21CN (ADSL 2+) it appears that BT have upgraded their network to support larger sizes - but then it's down to the individual ISPs (my ISP is Zen), I think. In any case, most operating systems these days use path MTU discovery to reduce fragmentation.

It appears (to me) that perhaps the router has already set the optimum MTU during negotiation of the PPP link, somehow. BTW, I've only tried setting the value through the web interface.

I would appreciate any comments on this.

Dave Robinson
Malvern, UK