PE-2650 query on full/half duplex

nathan r. hruby nhruby at
Thu Oct 14 07:20:00 CDT 2004

On Thu, 14 Oct 2004 phil_galligan at wrote:

> Hi All
> DELL PE.2650
> RedHat Enterprise_ES_ver_3.0
> Acting WebServer.
> issuing the command [dmesg] to the server informs me that eth0 is running
> on half duplex
> tg3: eth0: Link is up at 100 Mbps, half duplex.
> tg3: eth0: Flow control is off for TX and off for RX.
> I don't feel too worried about this (ie. Link is up at 100 Mbps), but for a
> number of reasons, it has been questioned.
> I've investigated a number of ways on how to force full duplex at startup,
> but I'm not confident that any rewards will be reaped.
> Is this something I need to worry about?


Many switches get auto-negotiantion wrong and thus the card and switch
choose the wrong duplex.  100:Half is OK, if that's what your switch
thinks also.  If your switch thinks 100:Full and your card think 100:Half
that can be an issue.  For small packets, this is often unnoticeable (I ran
a dns server with the wrong duplex for months before noticing it :) For
larger packets/transfers or higher loads, things will seem abnormally slow.

You can change the duplex with ethtool. (For instance: "ethtool eth0 speed
100 duplex full autoneg off").  In Red Hat you can do this at boot by
adding the "speed 100 ... off" portion if the command to a variable called
ETHTOOL_OPTS in /etc/sysconfig/network-scripts/ifcfg-ethN.

See /usr/share/doc/initscripts-7.31.16.EL/sysconfig.txt for full details
about this and many other fancy-pants options.



nathan hruby <nhruby at>
uga enterprise information technology services
production systems support
metaphysically wrinkle-free

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