Turning on Full Duplex

Robert G. Brown rgb@phy.duke.edu
Wed Dec 30 23:35:18 1998

On Wed, 30 Dec 1998, Joseph Lesko wrote:

> I'm having troubles getting full duplex to work. I have a Kingston
> KNE100TX card with the Digital 21140-AF chip.
> I added the following line to my conf.module:
> options tulip options=14 debug=1
> dmesg gives the following information:
> Linux version 2.0.36
> tulip.c:v0.90f 12/17/98 becker@cesdis.gsfc.nasa.gov
> eth1: Digital DS21140 Tulip at 0x6400, 00 c0 f0 31 37 bd, IRQ 12.
> eth1:  EEPROM default media type Autosense.
> eth1:  Index #0 - Media MII (#11) described by a 21140 MII PHY (1)
> block.
> eth1:  MII transceiver #1 config 3000 status 7829 advertising 01e1.
> eth1:  Advertising 0101 on PHY 1, previously advertising 01e1.
> eth1: Using user-specified media MII 100baseTx-FD.
> The light on the card that says FDX does not come on. The hub says it
> supports full duplex. What am I missing?

Hubs never support full duplex.  Only switches can support full duplex.
A hub echos/repeats/shares a single signal among all its ports, and
therefore no two hosts can be transmitting at the same time.  A switch,
however, permits A to talk to B while C talks to A -- any pair of ports
can be connected in either direction at any given time (as long as it is
one to one).

So, you could mean that you have a switch (all switches support full
duplex, I believe) in which case FD should work or you could have a hub,
in which case nothing you do with the card to make it FD will work
because it can't.

If you DO have a switch, chances are pretty good that you don't want to
pass any options at all to the card.  The switch and the NIC should
autonegotiate and end up 100BT/FD all by themselves.  At least my
KNE100's do on our switch, and my generic 21140 tulips do as well when
connected via crossover.  It is only very rarely that setting the cards
explicitly works better than what they choose "for themselves" in
negotiation with their hub or switch port.


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb@phy.duke.edu