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

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 07 Dec 2021 10:07:50 UTC

--- Comment #2 from Zhenlei Huang <zlei.huang@gmail.com> ---
So it is weird.

If VLANMTU is disabled on parent interface, MTU of VLAN interface will be
limited off by 4 automatically. The MTU of vlans should be 8996 in your case.

Try the following steps:

1. Disabling all vlan hardware offloading features in rc.conf:

ifconfig_igb0="mtu 9000 -vlanmtu -vlanhwtag -vlanhwfilter -vlanhwtso
-vlanhwcsum up"
ifconfig_igb1="mtu 9000 -vlanmtu -vlanhwtag -vlanhwfilter -vlanhwtso
-vlanhwcsum up"

2. reboot

For the 2nd step, you could also destroy cloned interfaces and restart netif

# ifconfig vlan0 destroy
# ifconfig vlan1 destroy
# ifconfig ... destroy
# ifconfig lagg0 destroy
# service netif restart

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