lagg of em0/em1 + VLAN = lower MTU?

[Karl Pielorz]

--On 10 July 2015 11:06 -0700 John-Mark Gurney <jmg at funkthat.com> wrote:

> Try bumping the MTU on the root em's by 4 (1504) before creating the
> lagg...

I had thought of that - but didn't want to try it (on the basis that out of 
all the other example config's I've seen - no one else has to, and it will 
break the lagg MTU).

Anyway I tried it, by:

 - Bump the MTU on the em interfaces before creating the lagg to 1504

 - The lagg created, inherits this 1504 MTU

That breaks the untagged IP on lagg0 - i.e. I end up with:

"
lagg0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1504
 
options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
        ether 68:05:ca:08:1d:3b
        inet x.x.x.x netmask 0xffffff00 broadcast y.y.y.y
"

That MTU of 1504 is going to break lagg0 for untagged traffic :(


 - However, the VLAN's setup on the lagg still default to 1496 MTU (despite 
the underlying interface lagg0 being MTU 1504)

If I 'manually' bump the MTU on the individual lagg0.x VLAN interfaces - 
they will go to 1500, but traffic is still 'broken'.

I can't 'reduce' the bad MTU on lagg0 either - i.e. 'ifconfig lagg0 mtu 
1500' (to try to put it back to 1500) fails with an ioctl error.

So sadly, I'm still stuck :(

Something is lopping 4 bytes off the MTU when it shouldn't need to. None of 
the example configs I could find were from 10.1-R (or with exactly the same 
cards) - but obviously this is just bread & butter lagg/VLAN stuff, so it 
should work?


-Karl