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From: "Lutz Donnerhacke" <lutz@donnerhacke.de>
To: <freebsd-net@freebsd.org>
Subject: AW: FreeBSD does not reply to IPv6 Neighbor SolicitationsNUD
Date: Tue, 5 Jan 2021 17:46:48 +0100
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> $ ifconfig re1
> re1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
>
options=8209b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,WOL_MAGIC,LI
NKSTATE>
>         ether c4:12:f5:33:c9:7c
>         inet 192.168.170.5/24 broadcast 192.168.170.255
>         inet6 fe80::c612:f5ff:fe33:c97c%re1/64 scopeid 0x2
>         inet6 2001:470:ecba:3::5/64
>         media: Ethernet autoselect (1000baseT <full-duplex>)
>         status: active
>         nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

There is another possibility: The address could be in a tentative or
duplicate state.
In such a state the device SHOULD/MUST NOT respond to solicitation messages.

This can be caused by a "proxy arp/nd" device, which responds to an address
in
the local neighbour cache with its own solication message. It might be even
a loop.
We had such cases in the past, so that the device does not activate the IPv6
address without disabling the NUD (was a windows server). For fixed
addresses it
should be save to disable the flag.

But this state should be reported by ifconfig.

So, did you try the latest stable?