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beyondtime avatar Server

Why does connectivity differ for straight and crossover cabling towards 1Gbit MDI-X capable switch (from Linux OS)?

cn flag
beyondtime

  1. There's no constraint with directly connecting a Realtek 1000/100/10Mbit transceiver towards a common router.

  2. There was no connection ('Could not activate connection: Connection 'Name of Lan connection' is not available on device eth0 because device has no carrier') with connecting a Realtek 1000/100/10Mbit transceiver through a straight wired ('patch') LAN cable.
    Changing from straight wired to crossover cabling enabled common LAN connectivity.
    Information from ethtool eth0 with eth0 up, ip address manually set and connection enabled on nmtui was

Settings for eth0:
    Supported ports: [ TP    MII ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Supported pause frame use: Symmetric Receive-only
    Supports auto-negotiation: Yes
    Supported FEC modes: Not reported
    Advertised link modes:  1000baseT/Full
    Advertised pause frame use: Symmetric Receive-only
    Advertised auto-negotiation: Yes
    Advertised FEC modes: Not reported
    Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                         100baseT/Half 100baseT/Full
                                         1000baseT/Full
    Link partner advertised pause frame use: Symmetric
    Link partner advertised auto-negotiation: Yes
    Link partner advertised FEC modes: Not reported
    Speed: 1000Mb/s
    Duplex: Full
    Auto-negotiation: on
    master-slave cfg: preferred slave
    master-slave status: slave
    Port: Twisted Pair
    PHYAD: 0
    Transceiver: external
    MDI-X: Unknown (auto)
    Supports Wake-on: ug
    Wake-on: d
        Current message level: 0x0000003f (63)
                               drv probe link timer ifdown ifup
    Link detected: yes
  1. With sending ethtool -s eth0 mdi-x on and ethtool -s eth0 speed 100 autoneg on duplex full connectivity is possible with crossover and straight wiring.
    On ethtool -s eth0 speed 1000 autoneg on duplex full nmtui can't enable a connection anymore for straight wiring (CAT5E standard).

All related, suggested similar questions had been regarded and additional research on 'MDI-X' connectivity settings on Serverfault.com was done.
This (sometimes or with some automatic system configurations resets, also appearing erratic) uncommon connectivity difficulty with (modern) 1Gbit switch hardware is not to be expected, therefore

What reasons are cause for this connectivity (MDI-X functionality?) constraints?

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vidarlo avatar
ar flag
vidarlo
Are you sure that both cables are OK? The fact that it works on 100Mb but not 1Gb suggests the cable may not be proper...
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beyondtime avatar
cn flag
beyondtime
Thanks for the hint, just replaced patch cable with borrowed similar type CAT5e cable, what might has solved 100Mb vs 1Gbit speed constraints for the patch cable. (Still not fully convinced, so it's for testing now. It might still be related to software settings to some extent or OS behaviour, since the other cable was on duty for years on other device without any sign of limitation, before it was moved and flexed newly since years.) Still in question: Why does a speed limited cable (defective wires?) not allow to negotiate or connect with lower speed on (previous situation) <=100Mbit?
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beyondtime avatar
cn flag
beyondtime
update: iperf3 for 'lower reliable' cable shows full 1Gbit bandwidth (940 Mbits/sec sender, 938 Mbits/sec receiver) on same switch is possible, so it's a situational/temporary rtl....->switch(MDI-X?, software settings?, packet configurations?)->router->www difficulty?
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