Redundant links between switches are good for availability but dangerous at Layer 2. Ethernet frames have no TTL, so a broadcast sent around a loop of switches circulates forever, and because switches flood broadcasts out every port, it multiplies at every switch. Within seconds a broadcast storm consumes all bandwidth and CPU, MAC address tables flap as the same source address appears on different ports, and the network grinds to a halt. Spanning Tree Protocol (STP), originally IEEE 802.1D, prevents this by logically blocking redundant paths while keeping them available as backups that come into use if an active link fails.
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