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I am studying CCNA and I am trying to learn the logic of the networking. In the CCNA book, it told me that sometimes disabling auto-negotiation is a good idea but I didn't understand much why and couldn't find a good explanation on google.

So, accorging to the auto-negotiation when the two connected device have different parameters for the speed and duplex, auto-negotiation chooses the best option to process the transmission. So why do we do disable it sometimes if it gives us the best option and in which situations disabling it is a good idea?

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With rare exceptions, disabling Auto Negotiation is not a good idea.

Auto Negotiation (AN) is mandatory for 1000BASE-T and faster. It should stay enabled generally. Disabling it makes a default node (configured with AN) fall back to half duplex - so forcing 100M full duplex on one side causes a link partner with default configuration to connect using 100M half duplex, causing a duplex mismatch.

As Tripko's already pointed out, trying to force a certain mode can leave you with 100 Mbit/s half duplex.

Disabling AN is only required and useful in very special situations. It should be handled very carefully because it is likely to create problems when swapping components, resetting settings and similar.

Some systems support Auto 10/100, Auto 1000 or similar settings which explicitly don't disable AN but limit the advertised modes to fewer (or just a single) than technically possible. These settings can be used without the common issues caused by AN disabling.

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  • Thanks so much for the clear answer.
    – Red And Black
    Mar 22, 2021 at 11:45
  • Can't upvote this enough. The thing is: setting fixed speed/duplex modes makes at least one end of the link half blind towards the error in the situation. Just had the case last week. ISP had 100FDX fixed, and counted late collisions and CRCs, but was too lazy to care and tell anyone. Our CPE defaulted to 100HDX and reported only "collisions", which are not considered as errors (because normal on HDX) by our and don't show up. User experience at that remote site was pretty bad... With auto/auto at both ends, at both link ends have a chance to spont the error.
    – Marc 'netztier' Luethi
    Mar 22, 2021 at 16:38
  • F... that ISP...
    – Zac67 ♦
    Mar 22, 2021 at 17:42
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It's important to understand that the cases where auto-negotiation should be disabled ARE VERY RARE. Many times study material for tests are a bit out of date. Virtually all equipment manufactured in the last 20 years is fully compatible with auto-negotiation.

As @zac67 and others have pointed out, early implementations had some incompatibilities. But unless you're working on equipment older than you are, you should always enable auto-negotiation.

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Auto-negotiation can fail to establish highest speed if equipment is from different vendors. Also if there are errors on the link speed will be dropped as well as duplex settings - so you may end up with 100 Mbps half duplex connection, where 1 Gbps is possible. As it is mentioned - sometimes, there are many specific cases and different devices where you may want to force 100 Mbps while more is possible.

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  • How do we detect those? How do we know that auto-negotiation will fail? Thanks so much for the answer btw.
    – Red And Black
    Mar 19, 2021 at 7:39
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    Auto-negotiation can fail to establish highest speed if equipment is from different vendors. is a thing of the past. Cisco implemented the first generation of AN in a different way than everyone else, creating incompatibilities. Cisco adapted more than 20 years ago, so incompatibilties shouldn't be expected outside ancient hardware plants.
    – Zac67 ♦
    Mar 19, 2021 at 8:06
  • @RedAndBlack you can't really, it just happens and you go "huh that's odd, autonegotiation should be working" and it isn't so you turn it off. I suppose.
    – user253751
    Mar 19, 2021 at 15:01

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