[TriLUG] Slightly OT: network switch question

[Bill Farrow via TriLUG]

On Wed, May 16, 2018 at 10:33 AM, Cristóbal Palmer via TriLUG
<trilug at trilug.org> wrote:
> In summary: cables are more likely to fail than you might guess, and you
> should test them routinely.

Ethernet cables use multiple pairs of twisted wires that carry
differential signals. Each pair operates independently from the other
pairs. When you plug a cable into a switch, the mdi/mdx
auto-negotiation process attempts to work out how many pairs are
connected and if it is a cross-over cable. I think it also sends some
carrier signals to detect what the other switch is capable of. If
there are only 2x pairs (4x wires) in the cable, the auto-negotiation
will select 10BaseT (10 Mbps) or 100BaseT, if it detects 4x pairs it
will select 1000BaseT. It also depends on what the the other switch
reports (advertises) it can do.

When a single wire in a twisted pair is broken it may still work,
sometimes, but is becomes very susceptible to noise (external and
internal). Sometimes unplugging and plugging fixes the problem because
it restarts the auto-negotiation which may detect the bad pair and
drop the speed down to 10Mbps. So please check your cable, or try
replacing it with a temporary cable.

You might want to try plugging one end into a Linux laptop and use
ethtool to get the low level ethernet PHY chip info:

MyBox:~# ethtool eth0
Settings for eth0:
       Supported ports: [ TP MII ]
       Supported link modes:   10baseT/Half 10baseT/Full
                               100baseT/Half 100baseT/Full
       Supported pause frame use: No
       Supports auto-negotiation: Yes
       Advertised link modes:  10baseT/Half 10baseT/Full
                               100baseT/Half 100baseT/Full
       Advertised pause frame use: No
       Advertised auto-negotiation: Yes
       Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                            100baseT/Half 100baseT/Full
       Link partner advertised pause frame use: No
       Link partner advertised auto-negotiation: Yes
       Speed: 100Mb/s
       Duplex: Full
       Port: MII
       PHYAD: 0
       Transceiver: internal
       Auto-negotiation: on
       Supports Wake-on: d
       Wake-on: d
       Link detected: yes