[Bug 270559] if_bridge: does not forward packets properly for vlan 1

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270559

--- Comment #8 from Philip Paeps <philip@FreeBSD.org> ---
Our if_bridge(4) really only implements the learning and forwarding parts of
the original 802.1D standard.  It's really only aware of VLANs because
if_vlan(4) puts the relevant tags in the mbuf.  It does not try to be a
complete implementation of 802.1Q-2014 (which incorporates bridging, VLANs and
some other stuff).

The comment introducing the VLANTAGOF(_m) macro is a little confusing if this
patch is applied as-is.  I'm also not convinced that this change is correct. 
VLAN 0 is not a valid VID but it's perfectly possible for a dot1q tag with
VID=0 to appear on the wire: it indicates untagged traffic with a PCP or the
DEI bit set.

I wonder if we should use 0xFFF rather than 0 as the magic number for untagged
frames.  0xFFF is reserved in 802.1Q-2014 for implementation use.

Did either of you test what happens if a packet with a dot1q header that only
contains a PCP and not a VID gets forwarded?

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