3750 not negotiating Full duplex with connected PCs

[moazzam.ali2]

Hi,

We have recently installed 48 10/100/1000 port 3750 switch and set the ports to AUTO. End user PCs connected to these ports are set to 100 Mbps and Full Duplex. But we have discovered that most of the PCs (may be all) are operating at HALF duplex although speed is OK at 100 Mbps.

Can someone advise why the switch ports set to AUTO cannot negotiate FULL duplex with the PCs.

thanks,

Ali

[sourabhagarwal · Accepted solution]

A common issue with speed/duplex is when the duplex settings are mismatched between two switches, between a switch and a router or between the switch and a workstation or server. This can occur when manually hardcoding the speed and duplex or from autonegotiation issues between the two devices and this is what happening in your case.

please hardset speed/duplex setting on 3750 ports to 100/full. it should resolve the problem.

check this URL to get more information.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml#SpeedDuplex_mismatch

hope to help ... rate if it does ...

[moazzam.ali2]

Thanks everyone for the valuable input. We were under the impression that keeping switch port to AUTO will make the port negotiate and set to whatever is set on the other side (PC). But after reading the Cisco article, we realized that this also comes under mis-configuration.

We have fixed the issue by setting same speed and duplex on both ends.

thanks,

Ali

[rolf.fischer_2]

Hi Ali,

this is a lesson we all had to learn sometime!

Maybe you prefer this article, it's written pretty understandable:

http://en.wikipedia.org/wiki/Duplex_mismatch

Regards

Rolf

[scottmac]

If your cabling is not up-to-spec (i.e., not terminated per EIA/TIA 568A or B) you can have this problem.

The issue is that there is too much crosstalk in full duplex mode, so the system drops to a half duplex mode.

Did you make the jumpers by hand, or are they "store bought"?

let us know

Scott

[tdrais]

It is not the cables. This is the correct way for this to fail when you configure it this way.

In short you have turned off the negotiation on one side by setting it to 100 full. The other side attempts to negotiate and not seeing anything goes to the default of half duplex.

It is the ones that are set to auto on both sides and get a duplex mismatch that you can suspect cables and even then it is unlikely.

[glen.grant]

This is normal behavior , you have created a speed duplex mismatch the way you configured the switches as auto . A switchport can always sense the correct speed but cannot sense the duplex unless the device at the far (pc nic) is also set as auto thus it will go to a default which is 100/half and thus is working as designed . You either have to hardcode the ports to match the pc nics or set the switchports as auto and change all pc nics to auto also . Unless there is some reason I would set pc nics as auto . You are not going to want to hardcode all 48 switches and it will just get screwed up the first time a user moves to a different switch .