duplex mismatch

damien2008damien2008 Member Posts: 45 ■□□□□□□□□□
Why is the router displaying a duplex mismatch (duplex half) when both router and switch were set to auto ?

‘When auto negotiation is set on both devices both agree to the fastest speed supported by both devices, plus use full duplex if supported’
Using GNS3 BTW.....


  • Magic JohnsonMagic Johnson Member Posts: 414
    It only goes full if speed = 1000mbps. It also defaults to half if auto-neg fails.
  • damien2008damien2008 Member Posts: 45 ■□□□□□□□□□
    I have just tried this again on real equipment and it worked ok - no duplex mismatch, both router and switch set to auto and both end up with duplex full, speed 100.

    Must be a GNS3 thing .......
  • Magic JohnsonMagic Johnson Member Posts: 414
    Doh my rule above is correct only if one side fails auto-neg!

    EDIT: Head truly up my rear end today. /facepalm
  • damien2008damien2008 Member Posts: 45 ■□□□□□□□□□
    Ha, no problem mate, was scratching my head myself this morning
  • Magic JohnsonMagic Johnson Member Posts: 414
    damien2008 wrote: »
    Ha, no problem mate, was scratching my head myself this morning

    That's the problem sometimes using ems/sims! I'd honestly just been over auto-neg again in my network warrior book last week, what a shambles!
  • theodoxatheodoxa Member Posts: 1,340 ■■■■□□□□□□
    For 10/100, if autonegotiation fails (though, it shouldn't if both sides are set to auto), the device will default to Half Duplex. Cisco doesn't support Half Duplex on GBICs per the SWITCH OCG (not sure if this applies to Gigabit UTP interfaces also or not) and 10 Gigabit Ethernet and above run ONLY in Full Duplex. A Duplex mismatch will cause all kinds of problems since one end thinks it needs to follow CSMA/CD and the other doesn't. A link won't come up at all if the speeds don't match.
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  • fredrikjjfredrikjj Member Posts: 879
    Known GNS3 issue. I've never had the issue with the 7200s though so you could use those, with the added benefit of being able to run IOS 15 code.
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