Spanning Tree

Spanning Tree Protocol (STP) Spanning Tree is one of the most exam-heavy topics in Network Access (20%) — expect both multiple-choice questions and simulation questions that require you to identify root bridges, predict port roles, and configure PortFast/BPDU Guard from the CLI. --- ## Why STP Exists Ethernet switches forward unknown and broadcast frames out all ports except the one they arrived on. In a network with redundant switch links, this creates broadcast storms — frames loop endlessly, consuming all bandwidth and crashing the network within seconds. Spanning Tree Protocol (STP) (IEEE 802.1D) solves this by logically blocking redundant links so there is exactly one active path between any two switches. If the active path fails, STP unblocks a redundant link — but only after converging through its port states. --- ## The Root Bridge Election Every STP topology begins with electing a Root Bridge — the switch that becomes the logical center of the tree. Election uses the Bridge ID (BID), which has two components: | BID Component | Description | |---|---| | Bridge Priority | A configurable value; default is 32768 | | MAC Address | Tiebreaker when priorities are equal | > Rule: The switch with the lowest BID wins the root bridge election. If priorities are equal, the lowest MAC address wins. Because priorities must be set in multiples of 4096, the values you'll see are 4096, 8192, 16384, 24576, 32768, etc. Practical tip: Always manually set the priority on your intended root bridge using: `` spanning-tree vlan 1 priority 4096 ` Or use the shortcut macro: ` spanning-tree vlan 1 root primary `` --- ## Port Roles Once the root bridge is elected, every…