Errata patches for the kernel have been released for OpenBSD 6.7 and 6.8. When an NDP entry is invalidated the associated layer 2 address is not invalidated.
Binary updates for the amd64, i386, and arm64 platforms are available via the syspatch utility. Source code patches can be found on the respective errata page: https://www.openbsd.org/errata67.html https://www.openbsd.org/errata68.html As these affect the kernel, a reboot will be needed after patching.
