Re: Issues with eepro100 and full duplex

On 13 Aug 2003, Devin Heitmueller wrote:

> I am having some difficulties with configuring the eepro100 driver to
> operate in full duplex mode.  I am running an unmodified eepro100 driver
> that ships with the 2.4.21-pre5 kernel.  Here is what I am seeing.

That's actually a modified driver from my perspective.  I'm guessing
what you mean is "the driver that is in the kernel.org 2.4.21-pre source
tree, not one munged by a distribution vendor".

> Using mii-tool, I configure the device for autonegotiate.  Mii-tool
> reports autonegotiate at 100 Full duplex.  The NetGear switch also
> reports 100 Full duplex.  So far so good.

Yes, Good.

> Then I use mii-tool to configure the card to disable autonegotiate and
> run at 100 full duplex.  Mii-tool reports that the card is running at
> 100 full duplex, but the switch now reports that the link is at 100 HALF
> duplex.

That's exactly what is expected.  You have just created a duplex
mismatch, which as a flawed configuration.

  Running some more tests reveals similar results, where
> regardless of setting the card to half or full duplex, the switch always
> reports the link at half duplex.

That's exactly the expected behavior:
   Forcing the speed must turn off autonegotiation
Forcing the duplex results in a non-standard-conforming link.
Full duplex on twisted pair is only standard-conforming when it has been
autonegotiated. 

> I ran the same test against a Cisco managed switch, and am seeing the
> same results.  In either case, it always runs half-duplex, unless I set
> the card to autonegotiate.
...
> Is there something in the Ethernet spec that would make this expected
> behavior?   

Yes.  If autonegotiation does not take place, and it won't with a forced
link, the link speed is sensed based on the link beat.
Ethernet _is_ CSMA/CD (informally "half duplex"), so you end up with a
half duplex connection.

-- 
Donald Becker				[email protected]
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