[Bug 292760] [bridge] MTU of bridge inteface limits MTU of bridge-based vlans in despite of vlanmtu flag

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292760

--- Comment #2 from vova@zote.me ---
Sounds as wrong assumption for me - MTU is L3 maximum transmission unit size, 
802.1Q header is L2 header, why it should affect MTU then?

let me assume that bridge handles both tagged and untagged frames: 

# ifconfig bridge1 create mtu 1500 192.168.0.1/24 up
# ifconfig bridge1.1 create 192.168.1.1/24 up
# ifconfig bridge1.1
  bridge1.1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
1496
        options=0
        ether 58:9c:fc:10:e2:e3
        groups: vlan
        vlan: 1 vlanproto: 802.1q vlanpcp: 0 parent interface: bridge1
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>

in that case I CANNOT use either bridge1 (untagged) or bridge1.1 (tagged) with
correct MTU (1500) 

if I'll set 1504 on bridge1 -> my packets sent from 192.168.0.1 will be
fragmented later on (as most of the internet is not capable to process frames
more then 1500)

if I'll set 1500 on bridge1 -> bridge1.1 will have MTU 1496 and normal frames
forwarded via bridge1.1 will be unexpectedly fragmented.

As I understanded ->  vlanmtu flag should tell inteface that any VLAN headers
should not be accounted in MTU calculations.

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