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My office ISP provides a 1 GBPS connection. When I connect my laptop directly to the router provided by the ISP, and connect to fast.com, I get a speed of approximately 100 mbps.

Our office network is routed through a switch. When I connect the ISP's router to the switch, and then connect my laptop to the office lan, I get speeds between 900 mbps - 1 gpbs.

This is completely unexpected and counter intuitve.

What could be causing this phenomenon? Is it a measurement error, and is there a more reliable way of measuring bandwidth (But then, google's speedtest produced similar results).

Or is there something else at play? Would be glad of any insights, as it will help me in subsequent configurations.

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  • Has any answer solved your question? Then please accept it or your question will keep popping up here forever. Please also consider voting for useful answers.
    – Zac67
    May 17, 2023 at 17:48
  • Afraid not. I've tried all suggestions, but none of them work.
    – ranban282
    May 18, 2023 at 11:34
  • Have you checked the indicated link speed when connecting directly to the ISP router?
    – Zac67
    May 18, 2023 at 12:06
  • That's what the question is about. It is ~100 MBPS, which is lower than 1GBPS, which is what I get via the switch.
    – ranban282
    May 18, 2023 at 12:34
  • Has any answer solved your question? Then please accept it or your question will keep popping up here forever. Please also consider voting for useful answers.
    – Zac67
    Jun 17, 2023 at 13:29

3 Answers 3

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While you don't provide much information to really answer your question - especially which link speeds are indicated - the most likely reason for that effect is a bad patch cable.

When not all four pairs required for 1000BASE-T can be properly linked, some NICs/switches may fall back to two-pair 100BASE-TX. This is common with the widespread Broadcom chipsets ("Ethernet@Wirespeed") but also possible with others. However, it's not a standard feature (as of IEEE 802.3) and should not be relied on.

Another, more obscure reason for that effect might be that your laptop as an end-node uses an MDI port to connect to the router (in contrast to a switch's MDI-X port), causing it to link with 100 Mbit/s only. Try a crossover cable or adapter for comparison. (Most modern ports use Auto MDI-X to link either way, but that is not mandatory and might not be given in every situation).

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    It may also be a power saving configuration issue. If the laptop is on battery when testing the direct connection but on the AC adapter in normal use with the switch connection, it may limit the link speed to save power according to a software policy on the computer. Ultimately, you probably don't want to connect directly as the switch is necessary for normal use so things seem like you can just leave them as they would normally be and move on unless you feel a real need to figure out why it doesn't work right when you set it up 'wrong'. Apr 15, 2023 at 21:19
  • @FrameHowitzer I tried again with and without AC adapter. Same results.
    – ranban282
    Apr 21, 2023 at 15:06
  • @ranban282 Check the link state on the computer when you connect directly to the router. If it is limited to 100 megabit on the link state then that is the best you will get. You will need to troubleshoot the computer and router to see why it links at 100 megabit instead of 1000 megabit. Apr 24, 2023 at 6:19
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maybe you can check ISP router mtu

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There can be a variety of factors at play, it would be a sensible idea to have a brief Wireshark capture to see exactly what is happening with the packets.

Considering you are getting speeds in excess of 100Mbps I suspect all the ports along the way negotiate correct speed. Some things to consider:

  • are you the only one on the network? If someone else is casually browsing while you're running the test they will compete for bandwidth with you. Given the difference you posted this seems likely
  • CPU / resources of devices along the way; are they all okay?
  • some network protocols (e.g. routing protocols, LLDP, CDP, STP) will take a small chunk of bandwidth as well; we're talking small numbers here but it still will add up to first point above

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