Redundant links between switches protect against failures, but at Layer 2 they create loops. Ethernet frames have no TTL (time to live), so a broadcast in a loop circulates forever, causing a broadcast storm, MAC table instability and duplicate frames. Spanning Tree Protocol (STP) prevents this by blocking just enough ports to leave a loop-free tree, while keeping the blocked links ready as backups. Rapid PVST+ (Rapid Per-VLAN Spanning Tree Plus) is Cisco's implementation of Rapid Spanning Tree (IEEE 802.1w) that runs a separate instance per VLAN, so different VLANs can use different roots and paths.
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