Cabling and media

Copper UTP dominates horizontal access runs. Higher categories tighten twist rates and shielding to support higher frequencies: Cat5e and Cat6 carry 1G to 100 m; Cat6a carries 10G to 100 m. Copper is cheap and carries PoE, but it suffers EMI pickup and the hard 100 m channel limit (90 m permanent link + 10 m patch).

Ethernet links are either point-to-point or shared media. A full-duplex link between a host and a switch port is point-to-point: only two transmitters exist, so collisions are impossible and both ends can send simultaneously. Legacy hubs create a shared media half-duplex segment where CSMA/CD arbitrates — one transmitter at a time, collisions normal. Modern switched networks are effectively all point-to-point full-duplex; a half-duplex reading on a switchport today is a fault indicator, not a design.

Verifying the physical layer

• show interfaces status shows speed, duplex and whether the link trained at the expected rate.
• An interface stuck at 100 Mb on a known-gigabit cable run suggests a bad pair or damaged termination — gigabit needs all four pairs.
• show interfaces counters such as CRC and runts point to physical-layer faults; verify media before blaming configuration.
• Fiber links that pass light but flap can indicate dirty connectors or the wrong optic type for the cable plant.

Common mistakes

• Assuming 'fiber is always faster' — optics and standard determine speed; MMF and SMF carry the same rates at different distances.
• Running 10G over Cat6 for the full 100 m — Cat6 10G reach is roughly 55 m; use Cat6a or fiber.
• Ignoring that gigabit copper needs all four pairs working — a single bad pair falls back to 100 Mb on many NICs.
• Using a crossover cable mindset on modern gear — Auto-MDIX makes straight cables work in most cases.