duplex mismatch on phone

Unanswered Question
Apr 26th, 2007


Can someone explain the following to me please.All our ip phones are deployed with their settings pc/sw config as auto/auto. Desktops are all on 100fullmb as are the 3750 switch ports they connect to. Mostly all is fine but sometimes I get duplex mismatches on syslog server. When I change phone settings to 100full it resolves. How come this happens on some and not others.

Also what is best practice as we are deploying in other offices now; I would have though everything set to 100full.

Thanks ,


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b.orth Fri, 04/27/2007 - 06:27

Hi Frank,

afaik, the best practice for setting switch ports is to use auto/auto.

Not sure why some of your phones seem to work and some don't. The way you describe your setup, all of your phones should have duplex mismatches.

If you hard code your switch to 100/full you have to hard code every device you connect to the network.

When a device or switch port trys to do auto but it does not get an auto reply back from the other end it set itself for half, assuming the other end is half.

Hope that makes sense,


FrancisHoban Fri, 04/27/2007 - 06:56

Thanks Bob.I am still a little confused as this only happens randomly so I am thinking why isnt it happeening all the time as you say.Are you saying it is best practice to have the swithches on auto/auto and pc's as well.

All our network is setup as below.

All access ports on 3750 switch stacks are set to 100 full.So sw connection on phone is auto/auto; this negotiates with 3750 to 100 full.

PC connects to phone port which is at auto/auto. If pc is set to 100full then it will negotiate to 100full.

dhingst Fri, 04/27/2007 - 14:31

If you manually set a port to 100/full then plug in a device set to auto/auto that device will default to 100/half. If you set one device manually you have to set whatever it connects to manually as well. Best practice is to set everything to auto negotiate and that will result in 100/full. In your case you have all devices with a duplex mismatch and you should be having lots of performance and voice quallity issues.

Take a look at this link for details on how ethernet port speed and duplex negotiation works.


scottmac Fri, 04/27/2007 - 19:24

Check and / or replace your cabling ... specifically the jumpers from the phones to the jacks.

If the jumpers were hand-made, I can pretty much give you a 90%+ confidence on bad cabling.

Same if the cables are crushed, kinked, twisted, pinched, stretched (or in other than near-perfect condition).

Try another cable, preferably a commercially made "store bought" cable.

Good Luck



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