From owner-freebsd-net@freebsd.org  Fri Jul 10 13:07:28 2015
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Date: Fri, 10 Jul 2015 14:04:44 +0100
From: Karl Pielorz <kpielorz_lst@tdx.co.uk>
To: freebsd-net@freebsd.org
Subject: lagg of em0/em1 + VLAN = lower MTU?
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Hi,

I've got a 10.1 box with a couple of Intel(R) PRO/1000 (82571EB) cards in 
it, e.g.

"
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
...
"

I've set these up as a fail-over lagg interface, lagg0:

"
lagg0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
...
 inet y.y.y.y
...
 laggproto failover lagghash l2,l3,l4
 laggport: em1 flags=0<>
 laggport: em0 flags=5<MASTER,ACTIVE>
"

This works fine. If I add a VLAN now to that, I end up with:

"
lagg0.10: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 
1496
...
inet x.x.x.x
...
        vlan: 60 parent interface: lagg0
"


The MTU on lagg0.10 has shrunk by 4 (size of VLAN tag). Is there a way of 
avoiding that?


-Karl