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updates and additional info
  • 2 environments, office and production at datacenter
  • Both environments have Cat-5e cables, a cable was replaced when it tested as bad with Broadcom's cable testing, and some others cables have been switch out during trial and error troubleshooting
  • Both environments have HP ProCurve gigabit switches
  • Office environment has Cisco 861w router which spec sheets say has 4 100mbps ports
  • Production environment has Cisco ASA 5510 router which has a mixture of gigabit and 100mbps ports. I am currently looking into what ports are in use.... Ok, the switch is reporting gigabit connection to router.
  • note: I do not know how a 100mbps router port affects nic modes if there is a gigabit switch between them

    • This has been a recurring issue for us both on our office network and production. It rarely matters since normal traffic rarely comes close to utilizing 100mbps but when doing vmware conversions it is particularly annoying for it to take 8 hours instead of maybe 2 due to this limitation. My laptop and one of our office servers are showing as gigabit on our switch and we are seeing such speeds, however another server and NAS we are testing goes down to 100mbps mode when going through our primary office gigabit switch by way of wall drops (same wall plate as laptop). When we hook up the NAS to a local "desktop" gigabit switch it switches to 1000mbps mode and tests as such speeds to the laptop. In production only half of the nics report as 1000mbps though they all should. Any insight into why this might happen or what should be taken into account when troubleshooting would be appreciated.

    dmr83457
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      have you tested the cabling? – Rex Nov 16 '13 at 19:49
    • Yeah, multiple cables involved in two environment and some switched out over time so I think that can be ruled out. Just bought a cheap cable tester when pikcing up another gigabit switch so will try that out later. I'll post updates as I go along. I am also about to add some potentially valuable info that could reveal how ignorant I am on networking, but I'm a web app developer in a small company that has no admin so go easy on me. – dmr83457 Nov 16 '13 at 20:32
    • How long are the cable runs from the switches to the PCs? Are you making your own cables? – MDMarra Nov 16 '13 at 21:22
    • The drops in office probably have cables that were made during installation. These are between about 20 to 60 feet long through the ceiling. One of the longest runs was connected directly to a desktop gigabit switch and the primary switch lights report the connection as gigabit. I don't believe production has any custom made cables and all are between 1 to maybe at the max 10 feet. – dmr83457 Nov 16 '13 at 21:42
    • It's been a few years, but just in case anyone has a similar problem -- this is almost always caused by an incorrect mapping of pins to pairs in the cables. – David Schwartz Dec 07 '16 at 05:07

    1 Answers1

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    It sounds like you are using auto-negotiation because you plugged in the NAS box and the line speed was reported differently. Wikipedia actually has a very good article on auto-negotiation if you are interested in the minituae ( http://en.wikipedia.org/wiki/Autonegotiation ). In an ideal world, everyone would follow the spec exactly and all the manufacturers would play nice and it would always work correctly. That is not always the case. That would be the last-ditch, I've checked everything else kind of thing.

    The reason that the other commenters are asking is that during this auto-negotiation process, the NICs will listen to the messages and chose the best possible. If the signal to noise ratio is too low, the NICs move to a lower speed until they can both agree. Long runs reduce the signal strength. Runs near ceiling lighting or any A/C can induce noise. The reason we use twisted pairs is to reduce that induced noise.

    All that being said, the cheap-o cable testers do not test signal to noise ratio usually just insuring that the pin-outs are correct. If you have a noise issue, there are higher end testers. Typically, this is consultant work because the testers are expensive. Check for crimps in the line or bends greater than 90 degrees. Check to see if A/C cable are wrapped with or near the affected NICs. Check to see if the runs are near overhead lighting. Draw a map of the office and geographically figure out if the low NICs are in a certain area.

    Alternatively, you can "nail" down speeds on most NICs i.e. using a manual assignment of 1G instead of a negotiated 100 though this opening a different box of worms. HTH

    Rookie
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