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.

  • also, have a look at udld May 27, 2016 at 21:31
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1 Answer 1


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.)

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

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