network-hardware-guide

read the port lights first

• No light on either end: no electrical connection. The cable, the plug, or the port is switched off or dead.
• A light on one end only: almost always the cable.
• Light on, but the speed light shows the lower speed: one of the four wire pairs is damaged. Gigabit needs all eight wires, 100 Mbit/s runs on four.
• A port that blinks amber on a managed switch is often blocked on purpose: by spanning tree after a loop, or by port security after too many devices.
• Move the cable to a neighbouring port before you blame the switch. Then move the device to another cable.

from the wall to the switch

• Every socket in a room ends on a numbered port of a patch panel. The number on the socket and on the panel should be the same, and often is not: test it and relabel.
• To find where a socket ends without labels: a tone generator and probe, or plug in a notebook and look for its MAC address on the switch.
• A managed switch tells you what hangs on each port: the MAC address table, and LLDP for phones, access points and other switches.
• Patch cables are stranded and flexible, the cable in the wall is solid and must not be bent back and forth. Never crimp a plug onto solid cable with plugs made for stranded wire, or the other way round.
• Keep a few spare patch cables in each common length. Most "dead ports" are a tired patch cable.

Finding a MAC address on a switch and configuring ports: switching and VLAN cheat sheet.

wiring a plug or a socket

• Pick one standard and use it everywhere. Mixing A at one end and B at the other makes a crossover cable, which nothing needs any more: current ports sort that out themselves.
• Keep the pairs twisted right up to the contacts, untwisted for no more than about a centimetre. The twist is what makes the cable fast.
• Cat 5e carries 1 Gbit/s over 100 m. Cat 6 does the same with more reserve and 10 Gbit/s over short runs. Cat 6A is what to install today: 10 Gbit/s over the full 100 m.
• Avoid cable marked CCA (copper-clad aluminium). It is cheap, breaks easily, and heats up under PoE.
• Shielded cable only helps if the shield is connected at the panel and the panel is earthed. Shielded cable installed carelessly is worse than unshielded.

testing a cable

• A simple tester with eight lights checks that every wire arrives on the right pin. It finds breaks, shorts and swapped wires.
• It does not find a split pair: all eight wires connected to the right pins, but twisted with the wrong partners. Such a cable passes the light test, links, and then loses packets at speed.
• Many managed switches can measure a cable from the port: where the break is, in metres. Look for cable diagnostics or TDR in the port menu.
• A certifier that measures the cable against its category is what an installer uses for acceptance. Ask for the test report when new cabling is handed over.

# what did the port negotiate?
ethtool eth0 | grep -E 'Speed|Duplex|Link detected'          # Linux
Get-NetAdapter | Format-Table Name, LinkSpeed, Status       # Windows

# Cisco-style switch: measure the cable from the port
test cable-diagnostics tdr interface gi1/0/5
show cable-diagnostics tdr interface gi1/0/5

fibre and transceivers

• Both ends need the same kind of transceiver, for the same fibre type and speed. SR modules are for multi-mode, LR modules for single-mode.
• Fibre links have two strands, one per direction. No link after patching: swap the two plugs at one end.
• Never look into a fibre or a transceiver, and never assume it is dark. The light is invisible.
• Dust is the enemy. Keep the caps on until the moment you plug in, and clean a plug with a proper cleaning pen, not with a shirt.
• Fibre does not like tight bends. A loop smaller than a coffee cup loses signal or breaks the glass.
• For short runs inside a rack, a direct-attach cable (two transceivers with a fixed copper cable between them) is cheaper and simpler than fibre.

What the plugs look like: connector identifier.

where access points belong

• On the ceiling or high on a wall, in the room where the people are. Not in the cupboard with the switch, not above a suspended metal ceiling, not behind a monitor.
• More access points at lower power beat one at full power. A phone that hears a loud access point far away answers with its own weak radio and is not heard.
• Connect every access point by cable. A wireless link between access points halves the speed for everyone behind it.
• On 2.4 GHz only channels 1, 6 and 11 do not overlap. On 5 GHz, use 40 MHz channels in an office with several access points, not 80.
• Walk the rooms with a Wi-Fi analyser app on a phone before and after. Look at signal strength where people sit, not next to the access point.
• Count the PoE power the access points need before ordering the switch: PoE budget calculator.

a cabinet you can work in

• Label both ends of every cable, and every port on the patch panel.
• Patch cables as short as the path allows, in a few fixed colours: one for uplinks, one for servers, one for everything else.
• Velcro straps, not cable ties. A cable tie pulled tight crushes the pairs, and cutting it open later cuts the cable next to it.
• Leave ports free on every switch and note which ones are uplinks.
• A photo of the cabinet after every change, stored with the documentation.

Plan the cabinet with the rack planner.

worth having in the bag

• A cable tester with a remote end, and a tone generator with probe.
• A crimping tool, plugs for stranded and for solid cable, and a punch-down tool for sockets and panels.
• Patch cables in three lengths, and two couplers to extend one.
• A USB network adapter, and a USB to serial console cable.
• A small unmanaged switch: it turns one working port into five during a fault.
• A label printer.