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During the autonegotiation process, network devices communicate and determine the highest mutually supported data rate. In the case of a network adapter supporting both 10 Gigabit per second (10Gb/s) and 1 Gigabit per second (1 Gb/s), the negotiation will typically select the highest possible speed that both devices support.

If the network adapter supports 10Gb/s and the connected device also supports 10Gb/s, and the cable quality and length are suitable for that data rate, the autonegotiation will likely result in a 10Gb/s connection.

However, if there are factors such as excessive cable length or poor cable quality that prevent reliable communication at 10Gb/s, could it be possible that the negotiation succeeds but the transmission rate is lower, for instance 8Gb/s?

Or in case the above factors occur, the network adapter will inevitably renegotiate with the connected device and agree upon a lower speed, such as 1Gb/s, that can be reliably supported over the given cable conditions?

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  • I have a feeling the underlying issue in play is actually perceived performance including system performance limitations (CPU, Disk, memory) and protocol overhead, etc. Roughly 8 Gigabit of measured application throughput over a single 10 Gigabit link is fairly common once you actually use it for real applications that require transactional data, encryption, connection setup and tear-down, etc. If the question is really 'why do I not get a full 10 Gigabit on a 10 Gigabit link?' there are excellent answers for that already available.
    – FrameHowitzer
    Jul 5 at 18:36

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With the notable exception of "Smartrate" ports supporting any subset of 2.5GBASE-T, 5GBASE-T, 10GBASE-T, Autonegotiation does not train the cable.

If the Fast Link Pulses used for AN make it over the cable, the link partners select the best mutually supported/advertised speed and try to link with that. With an inferior cable that may fail, making the link process fail overall. The process will just restart, there's no falling back to slower speeds. You might need to configure down the speed manually, but you should rather consider fixing the cable issue.

On a substandard cable, the link may complete but then perform poorly due to data corruption and subsequent frame drops.

Also, there are no intermediate link rates other than those defined by the Ethernet variants (ie. ..., 10G, 5G, 2.5G, 1G, 100M, 10M) and, as above, no fallback on link failure or excessive errors.

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  • Thanks, but to further clarify, could the two devices negotiate for 10Gb/s and then gain poor performance due to above factors?
    – Riccardo
    Jul 5 at 21:33
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    @Riccardo, the bandwidth would still be 10 Gbps, but there may be excessive application retransmissions due to frame errors. The bandwidth is still 10 Gbps, but the application throughput may suffer greatly. Ethernet just drops bad frames, but the application may ask for missing data.
    – Ron Maupin ♦
    Jul 5 at 22:08
  • That's what I was looking for!
    – Riccardo
    Jul 6 at 6:16
  • @Riccardo, the ethernet is still 10 Gbps; it is not any slower. You seem to mean application throughput, but that is off-topic here.
    – Ron Maupin ♦
    Jul 6 at 13:10
  • Thanks for clarifying
    – Riccardo
    Jul 6 at 17:42

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