Misbehaving host always uses the gw's MAC as destination: Violation of standard?

Question

We have an appliance that always uses the IPv6 gateway's MAC address as the link layer destination address for all its outgoing IPv6 packets, even when the receivers IPv6 address is on the same subnet. The network is IPv6 over Ethernet.

E.g.:

Appliance: 2001:DB8::2342/64, MAC 00:11:11:11:11:11
Gateway: 2001:DB8::1/64, MAC 00:AA:AA:AA:AA:AA
Client: 2001:DB8::4711/64, MAC: 00:22:22:22:22:22

When ever the appliance tries to send a packet to the client, it looks like so:

Source IP address: 2001:DB8::2342, Source MAC address: 00:11:11:11:11:11
Dest IP address: 2001:DB8::4711, Dest MAC address: 00:AA:AA:AA:AA:AA

instead of

Source IP address: 2001:DB8::2342, Source MAC address: 00:11:11:11:11:11
Dest IP address: 2001:DB8::4711, Dest MAC address: 00:22:22:22:22:22

Unfortunately we don't have access to the appliance routing table and our gateway is dropping packets which should be delivered to the same subnet they originate from. Therefore the appliance's connectivity to its local subnet is effectively unidirectional.

We are in correspondence with the maker of the misbehaving appliance, but the support engineer for the device is pretty unaware of how IPv6 works.

My questions are:

• Is it a violation to any standard if a host sends every IPv6 packet to the gateway, even if the recipient is on the same subnet? If so, which standards (RFC, BCP, IEEE, whatever) are relevant?
• Is the gateway misbehaving dropping those packets instead of sending them out to the interface they came in?

Answer 1

It's not a violation of the standard to send all packets to the gateway, but it is inefficient. Usually that behaviour is controlled from the router advertisement. If it contains the local prefix and it advertises it as on-link then the appliance should insert a route in its routing table to deliver that traffic to the local network directly. So before concluding that the appliance is at fault take a look at your router advertisements.

The gateway is at fault though because it definitely has a route and should deliver the packets, optionally sending a redirect to the sender to let it know it doesn't need to use the gateway. Some firewalls allow you to block intra-subnet traffic forwarding, so it might also be a configuration issue there.

Your traffic is breaking because of a combination of two issues, which are either bugs or configuration errors/choices.