Field guide · Networking & wireless
Spanning tree
What a switching loop actually does — and why it is instant and total rather than gradual — plus how spanning tree prevents it, how the root bridge gets elected, and the two port settings that turn an all-day outage into one dead port and a log line naming the culprit.
Plate 01Questions
Questions people actually ask
- What actually happens in a switching loop?
- A switch floods a broadcast out of every port except the one it arrived on. With a loop, that broadcast comes back, gets flooded again, and multiplies — there is no hop count on a layer-2 frame to stop it. Within seconds the links are saturated and the switches are too busy forwarding copies to process anything else. It is not gradual degradation; the network stops.
- What does spanning tree do about it?
- It builds a loop-free tree over a physically redundant topology by electing a root bridge and then blocking any port that would create a second path to it. The redundant link stays physically connected and logically shut, ready to unblock if the primary path fails. The point is that you get redundancy without a loop.
- How is the root bridge elected?
- Lowest bridge ID wins, and the bridge ID is priority followed by MAC address. Since every switch ships with the same default priority, the election falls through to MAC address — which usually means the oldest switch in the building becomes root, because older hardware tends to have lower addresses. Set the priority explicitly on the switch you want as root rather than letting the accident stand.
- What is the difference between PortFast and BPDU guard?
- PortFast skips the listening and learning states so an access port comes up immediately instead of taking around thirty seconds — without it, a PC can finish booting before its port forwards and fail to get DHCP. BPDU guard shuts the port down if a spanning-tree BPDU ever arrives on it, which is the signature of somebody plugging a switch into a desk port. They belong together on every access port: PortFast makes it fast, BPDU guard makes it safe.
- Should I use rapid spanning tree?
- Yes, on anything modern. Classic STP takes up to fifty seconds to reconverge after a topology change; rapid spanning tree does it in seconds. A switch still running legacy PVST is a finding in a handover review, and the change is usually a single command.
- How do I find the loop when it is already happening?
- Console into a switch rather than trying to reach it over the network, which will not work. Look at interface counters for the ports with impossible broadcast rates, and at the MAC address table for an address flapping between two ports — that flap is the loop's signature. Then shut ports until it stops, starting with anything patched recently.
Plate —Other guides
The rest of the section
- 01RJ45 Wiring: T568A and T568BBoth colour codes, pin by pin, side by side.
- 02RJ11 Wiring and Phone PinoutsSix positions, three lines, two colour systems.
- 03Keystone Jack TerminationStrip, seat, punch, snap — without failing the certification.
- 0466 Block WiringFifty rows, four columns, and where the bridging clips go.
- 05110 Block WiringLay the pairs in, press the connecting block on.
- 06Cat5e vs Cat6 vs Cat6a vs Cat8Speed, distance, and power decide it — not the bigger number.
- 07Fibre Types, Connectors and PolishRead the jacket, read the connector, don't mate blue to green.
- 08Subnetting, ExplainedWhere the numbers come from, and the meaning behind them.
- 09PoE Standards and Why It Won't PowerThe switch reserves the class, not the draw.
- 10Camera Lens, Field of View and Pixel DensityCovering the room is not the same as identifying a face in it.
- 11Patch Panels and Labelling to TIA-606A label that survives is worth more than a tidy rack.
- 12Rack Planning: Units, Depth, Weight and HeatIt fits vertically. That was never the hard part.
- 13Writing a Switch ConfigurationFrom factory default to a switch you would hand over.
- 14Networking and Cabling GlossaryThe terms, and what they mean on a job.
- 15POTS to VoIP: What BreaksThe lines nobody remembers until they stop working.
- 16Testing an Analogue Phone LineTip, ring, and the four numbers that tell you whose fault it is.
- 17Toning and Tracing a CableFinding one pair in a bundle of two hundred.
- 18Reading a Cable Certifier ReportWhat PASS* actually means, and which number to look at first.
- 19Which Cable Tester Do You Need?Three tiers, and the one most people actually need.
- 20Running an Office Floor RestackEveryone sits down Monday and their machine works.
- 21Cable Management for Desks and RacksTidy is a by-product. Serviceable is the point.
- 22Asset Tagging and CMDB AuditsThe scan is easy. Scanning it into the right record is the job.
- 23Cabling an Office Fit-OutHow many drops, run where, and what it takes to add one later.
- 24Parcel Locker Site SurveysWhether the unit fits, powers, connects — and gets in the door.
- 25VLANs and TrunkingAccess, trunk, tagged, native — and which one is the security hole.
- 26Reading a TracerouteWhich loss is real, and whose network it is in.
- 27DHCP, DNS and Helper AddressesWhy the new VLAN gets no addresses, and why it is always DNS.
- 28Wi-Fi Channel Planning and AP PlacementMore access points at lower power, not fewer at maximum.
- 29SMART Attributes That Predict FailureA drive can report PASSED while it is visibly dying.
- 30BitLocker: Keys, Recovery and EscrowThe key exists somewhere, or the data does not.
- 31Grounding and Bonding a Telecom RoomOne ground, one path, and no loops between rooms.
- 32UPS Sizing: VA, Watts and RuntimeThe number on the box is not the number you need.
- 33Mounting Displays on Any WallFind the structure. Everything else is a repair bill.
- 34Mounting IP CamerasHeight, angle, and the surfaces that ruin the picture at night.
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Rather it was just doneWe do this work
We do this work
as well as document it
Anyone who has watched a network die the moment somebody patched two ports together, and anyone commissioning a switch that should survive it.