Hardware

Spanning Tree: How Switches Stop a Loop

7:43Coming soon

Someone added a second cable between two switches, in case the first one failed, and the whole network went down in seconds. One broadcast frame did it, circling the two links and being copied at every pass. This lesson shows why a Layer 2 loop is fatal, and how Spanning Tree keeps the spare link without the loop: the switches elect a root bridge, each keeps its best path to it, and one port is blocked until it is needed.

You'll learn

Why a switching loop never ends: a frame has no hop count or TTL, so a flooded broadcast circulates forever
The three symptoms: broadcast storm, an unstable MAC address table, duplicate frames
BPDUs and the root bridge election: lowest bridge ID wins, priority first, then MAC address
Port roles: root port, designated port, and the blocked (alternate) port
Port states and timers in the original Spanning Tree: blocking, listening, learning, forwarding, and up to 50 seconds to recover
Rapid Spanning Tree (RSTP): the same election, recovery in about a second
Edge ports: PortFast and BPDU guard on ports that lead to hosts
The real thing: reading show spanning-tree, and why a blocked port there is not a fault
Exam corner: which of three switches becomes the root bridge

Chapters

0:00The sensible thing that broke the building
0:47A broadcast and two corridors
1:19Why it never ends: no expiry stamp
1:42Storm, unstable clipboard, duplicate crates
2:03The fix: elect a head depot
2:40Root port and designated port
3:02The chained door: blocked
3:23A corridor is cut: timers and port states
4:05Rapid Spanning Tree
4:21Edge ports: PortFast and BPDU guard
5:01The real names
5:51The real thing: show spanning-tree
6:27Exam corner
7:02Recap and next time

Exam objectives: CompTIA Network+ N10-009: 2.2 (spanning tree), 5.3 (switching issues: STP — network loops, root bridge selection, port roles, port states), 5.5 (basic networking device commands).

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