Switch / Server - speed / duplex

Switch / Server - speed / duplex

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Subject Author Date
Switch / Server - speed / duplex Russ 02-28-2007
Posted by Russ on February 28, 2007, 10:01 pm
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Hi,

I had a debate with some people at wok on the effects of setting the
speed / duplex at the switch, as opposed to auto negotiate. If for
example, the switch interface is set to 100Mb/Full Duplex that is the
only speed the the node/s connected on that port can connect at? Not
10mb/Half duplex, not 100mb/Half Duplex etc etc. Is this correct?

Also if someone could provide a link to where this is documented it
would be appreciated...

Thanks in Advance

R.


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Posted by BernieM on March 1, 2007, 2:46 am
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> Hi,
>
> I had a debate with some people at wok on the effects of setting the
> speed / duplex at the switch, as opposed to auto negotiate. If for
> example, the switch interface is set to 100Mb/Full Duplex that is the
> only speed the the node/s connected on that port can connect at? Not
> 10mb/Half duplex, not 100mb/Half Duplex etc etc. Is this correct?
>
> Also if someone could provide a link to where this is documented it
> would be appreciated...
>
> Thanks in Advance
>
> R.
>

Speed settings must match for the link to come up but duplex mismatches will
cause collisions. Duplex mismatches can go unnoticed if the link is not
under much load but will make it impossible to get much throughput.

I'm a Cisco 'groupie' so I'll direct you to this explanation.

http://www.cisco.com/warp/public/473/46.html#auto_neg_valid

BernieM




Posted by Rick Jones on March 1, 2007, 1:17 pm
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> I had a debate with some people at wok on the effects of setting the
> speed / duplex at the switch, as opposed to auto negotiate. If for
> example, the switch interface is set to 100Mb/Full Duplex that is the
> only speed the the node/s connected on that port can connect at? Not
> 10mb/Half duplex, not 100mb/Half Duplex etc etc. Is this correct?

> Also if someone could provide a link to where this is documented it
> would be appreciated...

My boilerplate on how 100BT autonegotiation is supposed to work:

How 100Base-T Autoneg is supposed to work:

When both sides of the link are set to autoneg, they will "negotiate"
the duplex setting and select full-duplex if both sides can do
full-duplex.

If one side is hardcoded and not using autoneg, the autoneg process
will "fail" and the side trying to autoneg is required by spec to use
half-duplex mode.

If one side is using half-duplex, and the other is using full-duplex,
sorrow and woe is the usual result.

So, the following table shows what will happen given various settings
on each side:

Auto Half Full

Auto Happiness Lucky Sorrow

Half Lucky Happiness Sorrow

Full Sorrow Sorrow Happiness

Happiness means that there is a good shot of everything going well.
Lucky means that things will likely go well, but not because you did
anything correctly :) Sorrow means that there _will_ be a duplex
mis-match.

When there is a duplex mismatch, on the side running half-duplex you
will see various errors and probably a number of _LATE_ collisions
("normal" collisions don't count here). On the side running
full-duplex you will see things like FCS errors. Note that those
errors are not necessarily conclusive, they are simply indicators.

Further, it is important to keep in mind that a "clean" ping (or the
like - eg "linkloop" or default netperf TCP_RR) test result is
inconclusive here - a duplex mismatch causes lost traffic _only_ when
both sides of the link try to speak at the same time. A typical ping
test, being synchronous, one at a time request/response, never tries
to have both sides talking at the same time.

Finally, when/if you migrate to 1000Base-T, everything has to be set
to auto-neg anyway.

--
The computing industry isn't as much a game of "Follow The Leader" as
it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

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