[c-nsp] full duplex mismatch speed - dynamips

[Jim Getker (getker)]

There is no such thing as gigabit half duplex, both link partners
transmit on all four pairs simultaneously.  So there can't be a duplex
mismatch when running at gig.  Auto-negotiation has to happen to link up
at 1000.  If you set a port to 1000 the phy is configured to not
advertise 10/100 but it does auto-negotiate.

Jim


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
Christopher.Marget at usc-bt.com
Sent: Saturday, August 21, 2010 11:37 AM
To: andrew at 2sheds.de; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] full duplex mismatch speed - dynamips

Andrew said:

>> On 20/08/2010, at 5:57 PM, <Christopher.Marget at usc-bt.com> wrote:

>> Two mentions of problems with manually configured gigabit operation.
>>
>> Is there really a problem in that scenario?  Shouldn't be.
>>
>> Regardless of what the UI appears to be doing, you can't do gigabit
without autonegotiating.  It shouldn't be possible to create a duplex
mismatch on a gig link.
>
>The issue is that the interfaces can usually do 10/100/1000.
>If one side is set to auto, and the other to 1000/Full - the side set
to auto will usually drop down to 10/Half, as this is usually the lowest
common denominator.

Thank you for underscoring my point about additional myths.  Now we have
another.

My original point: Duplex mismatches with equipment running at gigabit
(1000/full on one end, 1000/half on the other) just don't happen.  Or do
they?  I'm open to being corrected on this point, but only with an
explanation of how the (required) negotiation has gone wrong.

New myth: /Speed/ mismatches exist.  They don't.  You'll never see a
link operating with one end at 100 or 1000 Mb/s and the other end
running 10/half.

That's why we call them /duplex/ mismatches, not /speed/ mismatches.

Incidentally, the scenario outlined above will result in a correctly
operating 1000/full linkup because both ends (even the one set for
1000/full) are negotiating.

/chris (Adam's other new best friend)
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