[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 #9 from Aleksandr Fedorov <[email protected]> ---
The main problem is that the bridge doesn't set or inherit the VLAN_MTU flag.
Therefore, the VLAN driver lowers the MTU because it believes the parent
interface can't send packets with an MTU of 1500.

Quick and dirty patch, fixes the problem:

diff --git a/sys/net/if_bridge.c b/sys/net/if_bridge.c
index 9a468a8eb462..fe24991ad84f 100644
--- a/sys/net/if_bridge.c
+++ b/sys/net/if_bridge.c
@@ -871,7 +871,7 @@ bridge_clone_create(struct if_clone *ifc, char *name,
size_t len,
        ifp->if_softc = sc;
        if_initname(ifp, bridge_name, ifd->unit);
        ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
-       ifp->if_capabilities = ifp->if_capenable = IFCAP_VLAN_HWTAGGING;
+       ifp->if_capabilities = ifp->if_capenable = IFCAP_VLAN_HWTAGGING |
IFCAP_VLAN_MTU;
        ifp->if_ioctl = bridge_ioctl;
 #ifdef ALTQ
        ifp->if_start = bridge_altq_start;

# ifconfig bridge1 create mtu 1500 up
# ifconfig bridge1.1 create up
# ifconfig bridge1.1
bridge1.1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=0
        ether 58:9c:fc:10:05:32
        groups: vlan
        vlan: 1 vlanproto: 802.1q vlanpcp: 0 parent interface: bridge1
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@xenon:/home/XeNoN # ifconfig bridge1
bridge1: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric
0 mtu 1500
        options=18<VLAN_MTU,VLAN_HWTAGGING>
        ether 58:9c:fc:10:05:32
        id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
        root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
        bridge flags=0<>
        groups: bridge
        nd6 options=9<PERFORMNUD,IFDISABLED>

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