@ermal:

I will take a look as time permits.

Though i can make radvd to advertise the WAN link mtu rather than have an option to identify the mtu and configure by GUI?

Thanks for looking into this!

In this setup taking the WAN MTU would not really help. As the tunnel is on a PPPoE line, the best IPv4 WAN MTU would be 1492 and the best IPv6 MTU would be 1472 on the same interface. Standard settings for SixXS tunnels is an MTU of 1280 if you do not tinker with those.
In any case the MTUs for IP4 and 6 differ, so it would be the great if one could set the radvd MTU as an option.