lagg of em0/em1 + VLAN = lower MTU?

[John-Mark Gurney]

Karl Pielorz wrote this message on Mon, Jul 13, 2015 at 09:33 +0100:
> --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 :(

You can fix the untagged traffic on lagg0 with something like the
following:
route change <destination> <gateway> -mtu 1500

You can see which routes need to be changed w/ the netstat -rWnfinet
command...

> 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?

You should be able to make it work, but it definately needs more work
to make it work automaticly...  There was a recent change that went
in that makes changing lagg mtu easier, but I think that's only in
HEAD right now...

Hope this helps...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."