This one's a bit of a long shot, but DrayTek support haven't been very helpful so far.

Does anyone here have a native IPv6/IPv4 dual-stack internet connection, using a Vigor "IPv6 Ready" router?

I think Sky & BT both now have this enabled by default, if you were to set your IPv6 WAN mode to "PPP".

If so, and you're using it, do you run in DHCPv6(stateful) mode?

If so, are your DHCPv6 assigned hosts, able to ping other DHCPv6 assigned hosts?

In my test setup—which is nothing but IPv6 in PPP mode, and DHCPv6 mode on—the hosts have no way of communicating to each other. I know why this is, and have tried to explain it in a support ticket, but DrayTek seem to not understand what I'm saying.

When in DHCPv6(stateful) mode, the Flags section, in the Prefix Option, of the Router Advertisement has the "On Link" flag (L bit) set to 0/off, telling the hosts that nobody else with the subnet is on the Ethernet/WiFi link etc, and so packets destined to a host on the same subnet must be sent to the default gateway, rather than using Neighbour Discovery (ARP for IPv6).

So when a DHCPv6 addressed host tries to send a packet to another DHCPv6 addressed host, it sends the packet to the default gateway (as the host is told by the RA that the subnet isn't On Link). The gateway is then supposed to forward the packet on, and then inform the source host, using an ICMPv6 Redirect message, that the destination host is "on link", and can be contacted more efficiently that way.

The host will then add this information to its routing table to make future communication more efficient.

Alternatively, the Router Advertisement can tell the hosts that are hunting for a prefix, that other host addresses that share that prefix are directly connected, and so they can use ND to find them and talk with them. This is how I'd like to set my network up, given finer control over the Router Advertisements.

My DrayTek Vigor routers does this is SLAAC mode, but in DHCPv6 mode, it clears this L bit in the Prefix Option flags. But then it also does not forward packets on or use ICMPv6 Redirects when receiving a packet destined to the subnet it belongs to. Which breaks the network.


Anyone else notice this, or can test it on their own Vigor router?

I've just been using SLAAC on my unit as I found that my PCs used temporary addresses (which is what I want) when on LAAC but didn't on DHCPv6.

When in DHCPv6(stateful) mode, the Flags section, in the Prefix Option, of the Router Advertisement has the "On Link" flag (L bit) set to 0/off, telling the hosts that nobody else with the subnet is on the Ethernet/WiFi link etc, and so packets destined to a host on the same subnet must be sent to the default gateway, rather than using Neighbour Discovery (ARP for IPv6).

Sounds interesting so took a look at some RFCs. Isn't it slightly the other way around? I mean doesn't 0 mean that it's not given information rather than implying that it's off-link.

https://tools.ietf.org/html/rfc4861

Code:

L 1-bit on-link flag. When set, indicates that this prefix can be used for on-link determination. When not set the advertisement makes no statement about on-link or off-link properties of the prefix. In other words, if the L flag is not set a host MUST NOT conclude that an address derived from the prefix is off-link. That is, it MUST NOT update a previous indication that the address is on-link.

But then it also does not forward packets on or use ICMPv6 Redirects when receiving a packet destined to the subnet it belongs to.

Sounds strange if the router isn't passing on information though. Got any f/w rules which might be acting on LAN communication ?

Anyone else notice this, or can test it on their own Vigor router?

I'll take a look.

I tried this out (with a tunnel) and it seems to do exactly the same as described by @EeekiE. IMO there's a bug here mainly because of the discrepecy between SLAAC and DHCPv6 and also because of the ICMPv6 problem presumably caused by this.

FYI: I don't currently use the 2860 routers to route my ipv6, for all sorts of reasons my servers route ipv6 via HE tunnels. That said I have had it working but the tests were limited and my DHCP was still on my server. I did try it out to test this but did not keep it set that way.