bin/47364: dhcpcd resets mtu to 576 on Gbps

[george%galis.org@localhost]

>Number:         47364
>Category:       bin
>Synopsis:       dhcpcd resets mtu to 576 on Gbps
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Dec 25 22:10:00 +0000 2012
>Originator:     George Georgalis
>Release:        6.0 cdrom iso
>Organization:
>Environment:
NetBSD 6.0 (GENERIC) i386
>Description:
The install used dhcp fine but on first boot, dhcpcd (as configured by install) 
sets the wm0 mtu to 576 on a Gb wm0.

This doesn't completely break networking, but there are so many "discarding 
oversize frame" errors that it's pretty bad.

I don't understand why ifconfig cannot set the mtu but dhcpcd can???

More troubleshooting details on this thread
Date: Thu, 13 Dec 2012 20:18:27 -0800
Message-ID: 
<CAHK3FNzSDbZuOvUAakAjDkfYi+2KG2Ermk7ubjnf+dJn=mNzKg%mail.gmail.com@localhost>
Subject: discarding oversize frame
From: George Georgalis <george%galis.org@localhost>
To: netbsd-users%netbsd.org@localhost


>How-To-Repeat:
This problem was discovered on VirtualBox running on Windows 7 (don't get me 
started). Win7 OS reports physical card:
Intel(R) 82579LM Gigabit Network Connection 

The laptop is connected to (wifi and) a Cisco IP Phone 7975, which supports 
Gigabit and functions as primary gateway. I don't know what the phone is 
connected to but that switch probably has fibre to the backbone.

The problem persisted in both VirtualBox and VMplayer. It is unknown why there 
is not an issue for installer kernel/binaries with dhcp auto conf... The iso-cd 
network install was fast.

First boot in VMplayer also failed with mtu/framesize errors. When switched 
from installer written network dhcp config, to dhclient $int invoked from 
/etc/ifconfig.wm0 the problem was solved.
>Fix:
I won't be providing any network dumps but I can followup with config 
cause/effect tests if requested.