2 KNE100TX cards, no hub/switch, options= ?

Eric Jorgensen alhaz@xmission.com
Thu Apr 27 11:36:07 2000

Donald Becker wrote:

> > what should i be forcing?
> > 100Mbit ?  FullDuplex ?
> > at both ends? or just one (which one?) ?
> Neither.  You should allow autonegotiation to take place.  Read
>   http://scyld.com/expert/NWay.html
> forcing either the speed or duplex will prevent autoenegotiation.

	I agree with you, to a point, and I do not wish to contradict you. But
autonegotiation (not related to linux in any way) has caused me a great
deal of BS lately. 

	Autonegotiation is a good thing. You should only force a particular
state when two conditions are true: 

	1: Autonegotiation has failed and is causing a measurable degradation
in network throughput. 

	2: You are absolutly certian that you can do better (have read
manufacturer specs, etc) and know exactly how to force the state you
wish to experience on *both* ends of the connection. 

	That being said - I prefer autonegotiation, but sometimes it does fail. 

	Example: Cisco Catalyst switches and 3Com SuperStack II switches, when
network utilization is fairly high (over 30%), fail to autonegotiate to
full duplex. This caused me several hours of BS just this week. 

	Example: KTI 100baseTX to 100baseFX media converters fail to
autonegotiate when using two media converters on either end of a fiber
run where both ends are connected to 3Com SuperStack II switches.
3Com-KTI-KTI will negotiate 100 meg fdx, but when you add the 3Com on
the far end they will all re-negotiate to 10 meg half duplex. This
happens regardless of fiber length. KTI is aware of the issue and the
only fix is to aquire the model which is manually configurable. 

	Example: Xedia AccessPoint 100 CBQ bridge/router units aparantly only
autonegotiate with other AccessPoint units, or something. Xedia
technical support was initially baffled by my accusation that their unit
would not negotiate full duplex no matter what it was connected to, and
then later admitted that the only way their unit will negotiate to full
duplex is if the device the AP100 is attached to is already in full
duplex mode when it connects - ie, has been forced to full duplex. 

	I agree completely that in 99% of cases, you don't need it, and if it
fails, it probably wasn't ment to be anyway. 

	For instance, there are a lot of "switches" that aren't designed for
full duplex anyway. I have an Asante 12 port upstairs like that. Forcing
full duplex in this situation would be very bad juju. 

	If it doesn't work with a crossover cable, it might be a bogus cable,
try a different cable. You might have bogus nics, too. 

	But there are cases, such as high-utilization backbone connections,
where 100mbps full duplex is an absolute must, and when plug and pray
fails you simply have to force it. 

	You also have to be willing to do the research and make sure you know
what you're doing, and accept the fact that it might not work out the
way you planned and you might just have to find another way. 

	All that being said, Netgear FA-310TX's always autonegotiate full
duplex 100mbps for *ME* when i hook 'em up with a crossover cable. So do
cheapo D-Link cards with whatever that Maxonics chip is. (There's just
something *WRONG* with an $11 100baseTX nic)

 - Eric
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