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802.3 requires 1000BASE-T interface to do autonegotiation. But many network gear can disable autoneg and force 1000M-FDX operation for 1000BASE-T interface (such as "no autonegotiation / duplex full / speed 1000" ).

Despite that the spec requires autoneg and many gear venders also recommend to enable autoneg, my colleague insists on disabling autoneg. He prefer fixed configuration because autoneg-enabled 1000-T port sometimes "falls back" into 100/10M mode due to bad cable (for example one of 8 wires in Cat5 cable is cut).... Furthermore autoneg-disabled 1000BASE-T link in our lab is apparently working well.

My question: Does something wrong occur if I set "no autoneg" to each end of 1000BASE-T port? (e.g. BER increase or link flap ?)

AFAIK 1000BASE-X's autoneg includes remote fault signalling which detects uni-directional fiber cut. It makes senses to enable autonegotiation for all 1000BASE-X link.

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  • also, have a look at udld
    – Ronnie Royston
    May 27, 2016 at 21:31
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 14, 2017 at 4:28

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Autonegotiation should only be disabled where necessary. Properly compliant 802.3 hardware will send and respond to information in link pulses. Only in rare instances -- i.e. metro-ethernet -- are link pulses missing. And it is a violation of 802.3.

Setting speed/duplex does not disable negotiation; it limits what is advertised.

Note: The Cisco 2960S's at my fingers don't even support turning off negotiation. The Adtran hardware, luckily, does. (or I'd have no internet access.)

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  • Right, and for access switches you may have endpoints plug in that have various capabilities so you really want auto-negotiation.
    – Ronnie Royston
    May 27, 2016 at 21:25

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