Never force full duplex.

[Donald Becker]

On Sat, 1 Apr 2000, Metod Kozelj wrote:

> There's another consideration about FD vs. HD: if you have, say 4
> machines, all 100base-TX FD, and 3 of them want to push data to the fourth
> one full speed, the bandwidth to the fourth one is too narrow. Switch'
> buffers will become full and further packets from the first 3 machines 
> will get silently dropped. High-level protocol will have to do the
> re-transmissions after timeouts.
> 
> If all of them were HD, switch would insert collisions and low-level
> protocol would do re-transmissions, which is faster.

There is a relatively new feature, link flow control, that accomplishes the
same thing.  Many chips updated in the past two years have it.

It's negotiated in the same way as duplex, so forcing full duplex disables
it.  Even if you do remember to program your replacement switches with
forced-full-duplex, you are not getting the full potential of the hardware.


Donald Becker
Scyld Computing Corporation, becker@scyld.com

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