The basic problem here is BT's brain dead IPv6 deployment which only gives a dynamicIPv6 /56 address assignment when using BT Infinity. However, most routers I have evaluated recently (Billion and Netgear to mention two) seem to be able to request re-assignment of the delegated IPv6 prefix on PPPoE reconnect. I assume this done by remembering the previous assignment and requesting it in the DHCPv6 Solicit message.

However, when testing out a Vigor 2762, it seems to get a new dynamic IPv6 address each time it reconnects (reboot or otherwise). Is there an option that can tell the router's WAN DHCP client to remember and re-request the previous /56 or does Draytek simply compound BT's error by not supporting this capability?

Note: this problem seems to cause Linux boxes, in particular, a big problem as they end up accumulating global IPv6 addresses, adding a new one each time an RA message appears with a new delegated prefix.

I'm not sure that's what a DHCPv6 solicit message does - it's for finding the server not asking for a specific address. It may be that other routers just persist with the same address, and as it's in the same subnet, it still works...

RFC 3633 is where you need to look.

See https://tools.ietf.org/html/rfc3633#page-6

Prefix delegation is the process by which the ISP delegates an IPv6 prefix to the requesting router. The interesting text from the RFC is:

" A requesting router first creates an IA_PD and assigns it an IAID.
The requesting router then transmits a Solicit message containing an
IA_PD option describing the IA_PD. Delegating routers that can
delegate prefixes to the IA_PD respond to the requesting router with
an Advertise message.

The requesting router may include prefixes in the IA_PDs as a hint to
the delegating router about specific prefixes for which the
requesting router has a preference."

What I believe is happening is that the Billion and Netgear routers (that I have evaluated) remember the previous prefix delegated to them and then include it as the "hint" to the ISP in the (DHCP) Solicit message IA-PD option. The ISP will then normally delegate the same prefix. However, it seems that the Draytek router fails to do this. When the ISP always delegates the same IPv6 prefix then this is not a problem. However, when you get an ISP like BT that can't be bothered to permanently assign a IPv6 prefix to each customer, the Draytek router's behaviour just compounds the problem rather than working around it. IMHO, the renders the use of a Draytek router with BT Broadband problematic - and an increasing problem as IPv6 becomes more prevalent.

Sounds like it's working correctly to me, as you posted, the standard says that the requesting router can specifiy the prefix it requires, if your 2762 is requesting an address with a /56 prefix it could be allocated anything in the bold section of the following example 2001:0DB8:0000:00xx:hhhh:hhhh:hhhh:hhhh

If your allocated address always begins with the underlined blocks then it's correctly requesting a /56 prefix and then couldn't you add a static address from within this prefix to one of your 2762's interfaces?

No. The point is that the router gets allocated a different /56 every time it connects to BT. I believe that this is because it is not including the previous /56 in the DHCPv6 Solicit and hence BT always allocates a fresh one. The result is that none of my systems will ever have a stable global IPv6 address. I cannot otherwise force a static allocation as BT will only route traffic using the /56 it has assigned to the connection.

As i said, the underlying problem is that BT do not assign static IPv6 /56 unlike other ISPs - and yes, I will probably change ISP when my contract expires (e.g. to IDNet). However, I am making the point that other router vendors implement a workaround for this problem, while Draytek does not appear to.

But if the workaround is assuming an address in spite of what the server is telling you then that's breaking the protocol, no?