Auto Negotiation Duplex/Speed cause Flapping?

[elvinhokarhoe]

Hi,

I would like to find out is there any situation that auto-negotiation of duplex/speed will cause any flapping?

If it does, why? or does it used to have this issue and has been resolved?

Cheers,

Elvin

[johnlloyd_13]

Hi,

In my personal experience, I get a lot of user complaints saying that their connection is very slow because of speed/duplex mismatch.

Most of the time auto/auto on both end works just fine.


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[elvinhokarhoe]

Hi John,

My user insisted on setting manually thus I would like to know what is the con of setting auto and reason

Cheers,

Elvin

[kcnajaf · Accepted solution]

Hi Elvin,

My personal suggestion would be to use them manually if the machine is not moving very often (like a servers connected to a switch port and going to be on that port for a long time).

I would set this auto for an end user where there may be different type of machines which get connected to the switch port. Again if you are sure the end users machines are always configured with manual speed and duplex then i would do the same thing on the switch.

I have seen the scenarios where in the autonegotation sometimes does not work properly which results speed or duplex mismatch.

The rule of thumb is if you know the maximum speed supported by the server it always preferred to configured both speed and duplex on both server and switch side manually in order to avoid any conflicts.

Hope that helps

Regards

Najaf

Please rate when applicable or helpful !!!

[Daniel McDavid · Accepted solution]

I wouldn't recommend hard-coding speed and duplex for anything unless necessary. Auto-negotiation won't cause link-flapping. If is it, you need to test your run and make sure it passes. Try a different computer with the same run to see if it is the NIC. The user probably wants to hard-code it because they think it'll be faster, so just show them that they successfully auto-neg to a-1000/a-full so they are satisfied.

[Edwin Summers · Accepted solution]

Great article by Greg Ferro on why you should use auto-negotiation.


http://etherealmind.com/ethernet-autonegotiation-works-why-how-standard-should-be-set/

-Ed

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[markeelen]

Hi Elvin,
I'm not sure it's flapping you are seeing, I'm guessing you are seeing a slow link / no traffic at times them the link seeming to be working.
I've had to do quite a bit of research into this in the past, and my findings are as follows:
If you set speed and duplex at one end of a link, unless you set the other end the same, the link set as auto will default to 10 meg half-duplex.
This is the default operation of an auto port. If it cannot agree with the other end on speed and duplex, it will default to its slowest speed/duplex.
This will still 'sort of work' but you will see collisions on the link, especially if you have forced the other end into full duplex.
I found this out due to a manufacturer supplying a CCTV camera with its interface hard coded to 10 meg full duplex.
Have a look at the port you are connecting this machine to on the switch and do a show int counters errors, if there are loads of collisions, there's your problem.
Also, have a look here: http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml
Cheers
Mark

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[Joseph W. Doherty]

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Posting

If you set speed and duplex at one end of a link, unless you set the other end the same, the link set as auto will default to 10 meg half-duplex.
This is the default operation of an auto port. If it cannot agree with the other end on speed and duplex, it will default to its slowest speed/duplex.

Actually, speed/bandwidth can often be "computed" without auto negotiation (parallel detection), duplex, though, normally requires auto on both sides or both sides' duplex to be "hard coded" the same for Etherent and FastEthernet.  (Auto negotiation is a required part of the 1000BaseT standard.)

[markeelen]

Hi Joseph,
Have you tried labbing this up and seeing the results? When we were fault finding the CCTV kit, which was hard coded to 10 meg full duplex plugged into a 2960G, with the 2960 set as auto, the port always set itself as 10 meg half duplex, same results were seen with a 3550 and a 3750.
I think the document referenced above on packet pushers says it all, but, I have seen instances where auto negotiation fails between different vendors.
Cheers
Mark
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[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

If one side was hard coded full and other side auto, expected behavior is for auto side to set itself to half, just as you describe.

Yes, I've seen auto fail, but not so much on equipment manufactured in last decade and since.  (Different story when "auto" was new.)