if_em and large mtu bug?

[Pieper, Jeffrey E]

-----Original Message-----
From: owner-freebsd-net at freebsd.org [mailto:owner-freebsd-net at freebsd.org] On Behalf Of John-Mark Gurney
Sent: Thursday, May 31, 2012 11:52 PM
To: Bjoern A. Zeeb
Cc: freebsd-net at freebsd.org
Subject: Re: if_em and large mtu bug?

Bjoern A. Zeeb wrote this message on Wed, May 30, 2012 at 22:26 +0000:
> On 30. May 2012, at 11:36 , Eugene Grosbein wrote:
> 
> > 30.05.2012 17:44, Bruce Cran ??????????:
> >> Are there any known problems with if_em and jumbo packets? I've found 
> >> that my ssh connection breaks (when running dmesg etc.) if I configure 
> >> the mtu to 9216 via rc.conf - but if I initially use a 1500 byte mtu and 
> >> then manually configure it to 9216 then no problems occur.
> >> 
> 
> That is due to another known bug;
> if you check netstat -rn and the mtu there on your routes you'll figure.
> if you remove your addresses/routes and re-add them after changing the mtu,
> you'll see it as well.

Just to comment, this isn't a bug.. it's that FreeBSD allows you to
control the mtu to a host via the routing table...  Previously it would
always max out the routes MTU meaning that you could never reduce the
MTU to a problematic host, while keeping the mtu larger for others i.e.
have a mixed network w/ jumbo and normal frames...

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

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

This is what I use (I'm JFV's validation engineer):

Ifconfig emX <hostname> mtu <mtusize> 

Doing this adds it to the routing table and seems to avoid any issues. 

I hope this helps,

Jeff

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