[Bug 260260] igb(4) I35{0,4} parrent <--> vlan jumbo frame mtu mismatch

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260260

            Bug ID: 260260
           Summary: igb(4) I35{0,4} parrent <--> vlan jumbo frame mtu
                    mismatch
           Product: Base System
           Version: 13.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: zarychtam@plan-b.pwste.edu.pl

The vlan(4) children of LACP lagg(4) consistent of two ibg(4) I350 or I354 have
to use reduced MTU size to work.

To reproduce:

ifconfig_igb0="mtu 9000 up"
ifconfig_igb1="mtu 9000 up"
ifconfig_lagg0="laggproto lacp laggport igb0 laggport igb1 -lacp_strict"
vlans_lagg0="vlan0 vlan1 ..."
ifconfig_vlan0="inet x.x.x.x/y"

# iperf3 -R -c y.y.y.y
Connecting to host y.y.y.y, port 5201
Reverse mode, remote host y.y.y.y is sending
[  5] local x.x.x.x port 52750 connected to y.y.y.y port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.02   sec  0.00 Bytes  0.00 bits/sec
[  5]   1.02-2.02   sec  0.00 Bytes  0.00 bits/sec
[  5]   2.02-3.02   sec  0.00 Bytes  0.00 bits/sec
[  5]   3.02-3.55   sec  0.00 Bytes  0.00 bits/sec

#ifconfig vlan0 mtu 8996

# iperf3 -R -c y.y.y.y
Connecting to host y.y.y.y, port 5201
Reverse mode, remote host y.y.y.y is sending
[  5] local x.x.x.x port 49056 connected to y.y.y.y port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   118 MBytes   989 Mbits/sec
[  5]   1.00-2.00   sec   118 MBytes   990 Mbits/sec
[  5]   2.00-3.00   sec   118 MBytes   990 Mbits/sec
[  5]   3.00-3.69   sec  81.8 MBytes   989 Mbits/sec

There is no problem with sending jumbo frames, only receiving them is broken.
It is not hardware limitation, since bumping MTU on parents also solves the
issue and the configuration below is working fine:

ifconfig_igb0="mtu 9004 up"
ifconfig_igb1="mtu 9004 up"
ifconfig_lagg0="laggproto lacp laggport igb0 laggport igb1 -lacp_strict"
vlans_lagg0="vlan0 vlan1 ..."
ifconfig_vlan0="inet x.x.x.x/y mtu 9000"

The issue looks like either ibg(4) or maybe only I35{0,4} specific. I have more
machines with em(4) running similar setups, but only a few of them, those with
ibg(4) I35{0,4} NICs seem to be affected. Moreover, they all worked fine while
running either 11.4-STABLE or even 12.1-STABLE at the beginning of 2021.

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