Ran across something very odd today. Just had a symmetrical 100 Mb circuit installed at a site I’m contracting at.

When the NIC is set for auto negotiate for duplex and speed, we only get ~96 Mb down and ~45 Mb up. Same scenario if I set the NIC manually to duplex full and 1000. But if I set the NIC manually to duplex full at 100, we get the expected speeds of ~96 down ~96 up.

All cables are Cat6 and I tested this on a Mac, two windows machines, and a Cisco router. All behave the same with regards to the speeds. All tests were done directly connected to the edge switch.

I’m quite baffled. Not that it’s an issue, but why would 100 be faster than 1000? What would be the cause of this?

2 Answers 2


This is a buffering issue. I had the same problem with a new Level 3 circuit. Their NID apparently has no measurable buffers, so feeding it at a rate 10x the circuit rate (1000 vs. 100) leads to all manner of poor performance.

(They had the customer side set to auto (1000) but manually set the network side to 100. After a week of arguing, they set both sides to auto and the problem went away -- Charter/Spectrum, the actual carrier for the circuit, actually has a clue. Shocking, I know.)

Note: Cisco has no mechanism to control the autonegotiation process. There's no way to tell it to advertise 100/full. And autoneg is the only way to get full duplex -- unless you control both sides.

  • Heh, this is Level 3 also. Commented Jan 18, 2018 at 14:03

I would bet good money your provider has their side set manually instead. Its a poor and common practice for providers to rate limit a 100 Meg circuit. Being set to auto on your side would cause a speed/duplex mismatch which would explain the speeds you are getting when set to Auto. But set to 100/Full you match the provider equipment so everything is happy.

  • 1
    It auto negotiated to 1000 though. Should it not have auto negotiate to 100 then? If their switch port is manually forced to 100? Or is that strictly based on NIC and cable capability on the host side? Commented Jan 18, 2018 at 0:26
  • In my experience, auto-negotiation doesn't always work perfectly. It is increasingly rare, but some time it simply doesn't work between two devices and you have to set the ports manually. Poor / damaged cable also lead to this kind of issue.
    – JFL
    Commented Jan 18, 2018 at 8:55

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