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Community Member

Autosense Duplex and Speed issues with Cisco switches

G'day all

I manage a fairly diverse network that has a range of Cisco switches (2900, 3500, 3700, Catalyst 4500 etc). One consistent problem that I experience with the Cisco switches (and not with then cheapo Netgear etc) is Autosense speed and Duplex problems that cause response issues. I thought I had the problem licked by hard setting both the switch and PC cards 2 100/full. However I seem to be experiencing response issues again. When the PC is set to AUTO speed and Duplex it has improved response. Is there a hard and fast rule for setting this up? Surely someone else has experienced these sort of issues.

When I installed my new Catalyst 4500s nothing would work with any sort of reliability until I'd hard set all the interfaces.

I'm bamboozled.

Community Member

Re: Autosense Duplex and Speed issues with Cisco switches


Yes you have to define the speed in Catalyst 4500 Series Switch.By default its not set

In switches if the mode is Auto that means they are waiting for next hop to say the what mode to work on.

Like: Switch (Auto)<---------(Full)PC

To start comunication.

when you set switches on full deplex it will assume the following pc/switch is also 100mbps



Re: Autosense Duplex and Speed issues with Cisco switches

Hi there,

Just so you are aware - if you nail one side to full duplex and the other side to auto - it will always negotiate wrong and mismatch.

Hope that helps,


Please rate all posts


Re: Autosense Duplex and Speed issues with Cisco switches

This can sometimes / occasionally / frequently be tied to bad or improperly terminated cabling.

For example, if the (hand made) jumper cable was terminated instead of accordsing to the EIA/TIA 568 spec (either a or b), then you end up with a split pair on pins 3&6.

The split pair causes severe crosstalk on full duplex connections. The hallmark symptom of split-pair 3&6 is that the communication is ~ok-to-good at half duplex (or only works at 10Half), but fails or slows down significantly at full duplex (because of the crosstalk interfering with the concurrent signaling).

If your jumpers are hand-made, try a couple "store bought" cables and see if te problem persists. If you for some reason decided to not follow the color code on the panel punch-down blocks or info outlets, it would be worthwhile to re-punch according to the spec.

Some older flavors of Cisco switches didn't do hard-set values well. At the least, some complained abot duplex mismatch (but it was really OK and not service affecting) ... some actually did have problems and slowed things down, some work(ed) OK.

Usually, try it one way, if you get errors, try the other way ... if you *still* get errors ... start swapping cabling and scanning the Bug Board at the main Cisco site and think about up/down grading the switch OS.

Good Luck



Re: Autosense Duplex and Speed issues with Cisco switches

As everyone has said make sure both sides are the same . You don't mention the flavor of the 2900's or 3500's if they are the old 3500's or 2924's make sure you are running the latest code for those . If your nics are old in the pc's the drivers may need to updated . We have never had very many problems with the either leaving them as auto or hardcoding the ports .

Community Member

Re: Autosense Duplex and Speed issues with Cisco switches

I would say the answer to your question is no, there isn't one rule for the duplex/speed settings. I have experienced issues with port settings both hard coded AND auto.

I would suggest choosing the setting on the switch port that compliments the NIC setting in the PC, not forcing the PC to adjust to the switch port setting. Upgrading NIC drivers can help as well.

Community Member

Re: Autosense Duplex and Speed issues with Cisco switches

I have a question along this same line. I have gig ports on my Cisco 6509Es and gig ports on the servers. I am leaving them at Auto/Auto on both sides and everything seems to be working great. My question is, " I thought that I read somewhere that Cisco recommended gig trunk ports and pc to switch port speed and duplex settings be left at auto/auto". What are your thoughts on this? Especially on trunking.

I always hard set my 10/100 NICs and ports to full/100 but I was under the impression that with gig I no longer needed to do that. Am I barking up a tree on this one?



Re: Autosense Duplex and Speed issues with Cisco switches

We leave all our gig switch to switch connections as auto and have never had a negotiation issue with the links . We haven't had a lot of issues even with end user stations leaving them as auto . The problems usually arise when you get a user who thinks they know networking so they try to harcode the nic card which then results in a speed/duplex mismatch if the switchport is at auto .

Community Member

Re: Autosense Duplex and Speed issues with Cisco switches

FWIW: I've never had any issues, given correct and good-quality wiring, when using 'Autonegotiate' at both ends.

I have, on the other wing, experienced nasty performance hits when the server or workstation end is nailed to a fixed speed/duplex, and the Cat-5505 port is set to 'Auto.'

Suggest making sure everything in your LAN is set to autonegotiate. It really works pretty well with the current crop of NICs.

Keep the peace(es).

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