IPv6 Neighbor Discovery Issue

Hello,


I am currently facing the following problem:

I have a server with two interfaces:

- em1 (Outbound / facing the Internet)
- em0 (Internal use / LAN)
(additionally: vlan1000 - parentdev is em0)

The server runs OpenBSD 6.6-stable with the latest
syspatches installed and rebooted to the patched kernel.


I assigned one address of the /56 my provider gave me
to em1. Let's say it looks like this (obfuscated):


Prefix = 2a02:aaaa:bbbb:cc00::/56
Gateway (provider) = 2a02:aaaa:bbbb:cc00::1


I assigned to em1:

2a02:aaaa:bbbb:cc00::2/56


### /etc/hostname.em1 ###

inet6 2a02:aaaa:bbbb:cc00::2 56
!route add -inet6 default 2a02:aaaa:bbbb:cc00::1
-soii

-----------------------


This far, everything works (inbound and outbound IPv6 connectivity).



In the next step I took a /64 from that range and assigned an address to
the vlan1000 interface:


2a02:aaaa:bbbb:ccff:dead::1


### /etc/hostname.em0 ###

up

-----------------------

### /etc/hostname.vlan1000 ###

inet 10.20.30.40 255.255.255.0 vnetid 1000 parent em0
inet6 2a02:aaaa:bbbb:ccff:dead::1 64

-----------------------


This worked "somehow" (e.g. for a short period of time)
"Worked" means: This address was reachable from the outside world
and vice versa.

After it stopped working I did a reboot and then it worked again
(for a limited amount of time)


All i can see (from tcpdump) is that the provider gateway sends NDP
solicitations, asking for 2a02:aaaa:bbbb:ccff:dead::1
But no replies are appearing.

The same behaviour occurs independently from either

- pf disabled
or
- allowing anything IPv6 related (icmp-v6 etc.)


I already enabled net.inet6.icmp6.nd6_debug but nothing shows up in
dmesg.



Has anyone encountered this as well and/or has hints on
how to solve this?


Thank you very much for your time.


Best regards,

Alex