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I have a point to point connection by fiber with 1000BASE-X Ethernet. The rate is always at 1 Gbps, and I believe full-duplex would not be a problem.

Since the link is point to point, it seems that there is no reason to "negotiate" speed or duplex.

Therefore, can I just disable the Auto-Negotiation function?

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  • There is no negotiation on fiber ethernet connections; it connects at 1 Gbps, or it doesn't. Negotiation really isn't any issue since, even on copper interfaces, it only happens right when the interface comes up.
    – Ron Maupin
    Mar 30, 2017 at 22:21
  • Great! That means I can disable Auto-Negotiation and not worry about it.
    – Abe
    Mar 31, 2017 at 0:15
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    Why bother? You will never notice it, it does nothing bad, and there are things that may expect it. In networking, as in many things, "If it ain't broke, don't fix it!" is excellent advice.
    – Ron Maupin
    Mar 31, 2017 at 0:52
  • 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 can provide and accept your own answer.
    – Ron Maupin
    Feb 19, 2018 at 3:51
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    "There is no negotiation on fiber ethernet connections" — this is incorrect. Clause 37 defines Auto-Negotiation for 1000BASE-X fiber interfaces. While it is true that the speed is fixed at 1000 Mbps, duplex and flow control can be negotiated and remote faults can be indicated.
    – claymation
    Jan 13 at 1:25

4 Answers 4

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The link probably doesn't need auto-negotiation, but you probably should not change the default without a good reason. There is nothing about having it configured that will cause you any problem, and it only happens when the link comes up. It takes virtually no time to happen. You really don't want to make network changes, just because. "If it ain't broke, don't fix it!"

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If your network now are working well, you don't have to change it.

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    Can you expand this answer a bit more, it's rather vague.
    – Baldrick
    May 5, 2017 at 14:39
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If you disable negotiation then you must do it at both ends as it is not Auto-Detect. Negotiation means the two ends negotiate the connection that is they talk, so If you change only one end the other end will attempt to negotiate get no answer and decide what to use on its own. With 100Meg ports this is always 10Meg, Half duplex as (IMHO) that is the most likely if the device does not negotiate. The bad thing is that with Full/Half with a lightly loaded link it still works so you move on and when lots of traffic hits the link it goes down.

I've not tried this on 1000 Meg or 10000 Meg links so I don't know if this still happens. I'm not really sure how full/half duplex applies to a 1000Meg link - as on copper I thought it used all four pairs to transmit - so it seemed to me to be inherently half-duplex but I'd think it is point to point and negotiates who will talk next that is not collision sensing. If so then the Half duplex setting would have a different meaning and you might find it cannot be applied in the config.

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    "With 100Meg ports this is always 10Meg, Half duplex as (IMHO) that is the most likely if the device does not negotiate." — this is incorrect. Clause 28 specifies a Parallel Detection mechanism by which a link partner with Auto-Negotiation can establish link with a link partner without Auto-Negotiation. The link partner using Auto-Negotiation can detect the speed of the link partner, but must assume half duplex (since it can't detect that).
    – claymation
    Jan 13 at 1:28
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    "The bad thing is that with 100M Full/10M Half with a lightly loaded link it still works so you move on and when lots of traffic hits the link it goes down." — this is also incorrect. It is not possible to have a link that one link partner perceives as 100 Mbps and the other link partner perceives as 10 Mbps. The speed must always agree — the duplex can be mismatched, which does cause the kind of problems you're describing.
    – claymation
    Jan 13 at 1:30
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Therefore, can I just disable the Auto-Negotiation function?

You could but you shouldn't. You should leave Auto Negotiation enabled at all times. There are very few scenarios where it makes sense to disable it.

With Ethernet over twisted pair, disabling AN is a common mistake that sooner or later lands on your feet unless properly documented. It's mandatory for 1000BASE-T and faster anyway.

By IEEE 802.3, Auto Negotiation for 10/100 Mbit/s and fiber is still optional, so hardware often allows disabling it.

In practice, it's not a bright idea to do so unless you've got ancient, non-compliant hardware and know exactly what you're doing - manual configuration always has to match on both sides of the link (speed/duplex/pause mode). Mismatching the speed won't bring up the link, but mismatching duplex modes enables the link but makes it perform extremely poorly.

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