I just swapped out a Vigor 2820n with a 2860n (to take advantage of some of the new VPN features). It worked well except on a few higher powered systems running Fedora 19 Linux.

Running Wireshark I was seeing huge numbers of TCP Retransmission errors and whilst internal network comms were good, anything going out over the internet was pretty slow.

After pulling the network cable and reinserting it, then looking at /var/log/messages
it shows the IPv6 connectivity start first - then failing and then attempting IPv4 which it succeeds with a normal class B IP address

Solved!
It turns out that the Vigor 2860 comes with IPv6 compatibility and Fedora Linux NetworkManager sets up IPv6 by default which had been OK with the Vigor 2820, but when replaced with the 2860 that supports IPv6, the two don't play well at all, and disabling IPv6 on the Linux boxes sorted out the problem instantly. To do this I just clicked the IPv6 tab in NetworkManager and changed the setting from Automatic to Disabled.

But not Quite Solved!
So surely, this should have worked and the 2860n DHCP should have just issued an IPv6 address... So anyone know why it didn't work?

IPv6 does not use DHCP but default, but rather the linux machine should of figured out its own address using stateless auto configuration using the routing prefix provided by the Drayteks Router Advertisements.

Yeah v6 is a bitch.