[Bioclusters] Xserve going full duplex

Donald Becker bioclusters@bioinformatics.org
Tue, 12 Aug 2003 10:31:15 -0400 (EDT)

On Tue, 12 Aug 2003, andy law (RI) wrote:
> Donald Becker wrote:
> > That is a very bad solution.  If you must must disable autonegotiation
> > because of flawed hardware, disable the broken end and allow 
> > the link to use half duplex.
> > Bottom line: Auto-negotiation works.  If you turn it off because of
> > broken switches, never set the port forced-full-duplex mode.
> But from reading Table 1 in the article on the cisco site, if the
> problem is at the switch end,

 And it usually is..

> the *ONLY* way to get a 100MB connection
> is to manually configure both ends.

The transceivers on almost all boards will fall back to autosensing the
link speed if autonegotiation fails.  If the link speed and duplex is
configured manually, autonegotiation is implicitly turned off.  The 
link partner will assume a 100Mbps CSMA/CD ("half duplex") link if it
detects a 100baseTx link beat.

> Now I'm not au-fait with the reasons to choose half-duplex or
> full-duplex so I could be missing the point, 
> but I do know that if it comes to a choice between a 10MB link, a 100MB
> link with a mismatched duplex or a working 100MB link then I'll choose
> the 'bad solution' of manual configuration. 

That is a flawed statement of the choices.  The choice is between
   Always configuring equipment to forced 100baseTx FULL duplex
      (Cisco's old advice, reinforced by years of training courses)
   Using autonegotiation by default, and configuring the older flawed
      switches to forced 100baseTx half duplex

A half duplex link performs within a few percent of a full duplex link
for most applications.  A link with mismatched will work without
apparent problems when lightly loaded, but have severe, mysterious
problems when the workload increases.

If you follow Cisco's old advice of forced full duplex, you've just
committed your installation to long-term administrative problems and
expense.  Every piece of equipment that is added or moved must be
manually configured or verified by a knowledgeable person.  (The cynical
can read the last as "trained CCNE".)  And increasingly you will
encounter problems with equipment that cannot be manually configured.

If you use autonegotiation mixed with 100baseTx half duplex, you never
have to configure new or moved equipment.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993