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Routing & Switching · Foundational

Spanning Tree Protocol (STP)

IEEE 802.1D protocol that prevents Layer 2 loops by electing a root bridge and blocking redundant paths.

Definition

Switches exchange BPDUs to elect the root bridge (lowest bridge ID), choose a root port on each non-root switch and a designated port per segment; remaining ports block. Classic STP convergence takes 30-50 seconds using 20-second max age and 15-second forward delay. Modern networks use RSTP or MST.

Example

show spanning-tree vlan 10

Exam tip

Layer 2 loops cause broadcast storms, MAC table instability and multiple frame copies, because Ethernet has no TTL.

Don't confuse it with

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