vtnet with vlan doesn't work without promiscuous mode

Hello!

I have a virtualbox FreeBSD 15 machine.

I have a bridged interface (bridge in Virtualbox) to a physical trunk
interface on a Debian host. If I boot Fedora in such a situation, that
works fine - I can use the tagged VLANs from inside the VM.

On FreeBSD, that only works if promiscuous mode is enabled for the
virtual vtnet0.5 nic (that enabled promisc-mode for vtnet0 too).

If running without it, I do not see the incoming NDP multicast traffic,
outgoing is fine

I verified that using ifconfig and starting tcpdump with and without 
--no-promiscuous-mode.

The first issue is that incoming IPv6 NDP multicast traffic is not being
seen when promiscuous mode is off, if I enable it, all is fine.

I confirmed that the system is listening on the multicast addresses
(regardless of promisc), in my case ff02::1:ff6c:2017.
The corresponding multicast MAC is also there:

[m@test ~]$ netstat -na -I vtnet0.5
Name       Mtu Network               Address
   Ipkts Ierrs Idrop    Opkts Oerrs  Coll vtnet0.5  1500 <Link#4>
       08:00:27:6c:20:17                       307     0     0       64
    0     0 33:33:00:00:02:02      261             24 33:33:93:ec:db:5b
     261             24 33:33:ff:93:ec:db      261             24
                          33:33:00:00:00:01      261             24
                          33:33:ff:6c:20:17      261             24
vtnet0.5     - fe80::%vtnet0.5/64    fe80::a00:27ff:fe6c:2017%vtnet0.5
      30     -     -       60     -     - ff02::202%vtnet0.
                          ff01::1%vtnet0.5 
                          ff02::2:93ec:db5b
                          ff02::2:ff93:ecdb
                          ff02::1%vtnet0.5 
                          ff02::1:ff6c:2017
vtnet0.5     - 2a01:170:118f:5::/64  2a01:170:118f:5:a00:27ff:fe6c:2017
       0     -     -        0     -     - ff02::202%vtnet0.
                          ff01::1%vtnet0.5 
                          ff02::2:93ec:db5b
                          ff02::2:ff93:ecdb
                          ff02::1%vtnet0.5 
                          ff02::1:ff6c:2017
[m@test ~]$ 

How can I further diagnose that?

I've already checked the situation with Fedora in another VM in bridge
mode, no promiscuous mode is needed to communicate, so I assume
this issue is in the FreeBSD VM.

Please let me know if you need further info.

-- 
kind regards
Marco

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