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Cisco® Field Manual: Catalyst® Switch Configuration by Steve McQuerry, Dave Hucaby

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7-1. STP Operation

  • STP detects and prevents Layer 2 bridging loops from forming. Parallel paths can exist, but only one is allowed to forward frames.

  • STP is based on the IEEE 802.1D bridge protocol standard.

  • Switches run one instance of STP per VLAN with Per-VLAN Spanning Tree (PVST). PVST between switches requires the use of Inter-Switch Link (ISL) trunking.

  • For IEEE 802.1Q trunks, only a single instance of STP is allowed for all VLANs. The Common Spanning Tree (CST) is communicated over VLAN 1.

  • PVST+ is a Cisco proprietary extension that allows switches to interoperate between CST and PVST. PVST bridge protocol data units (BPDUs) are tunneled over an 802.1Q trunk. Catalyst switches run PVST+ by default.

  • Multiple Instance Spanning Tree Protocol

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