The Need for Spanning Tree

STP prevents three common problems in Ethernet LANs that would occur if the LAN were to have redundant links and STP were not used. All three problems are actually side effects of the fact that without STP, some Ethernet frames would loop around the network for a long time (hours, days, literally forever if the LAN devices and links never failed).

Just one frame that loops around a network causes what is called a broadcast storm. Broadcast storms happen when broadcast frames, multicast frames, or unknown-destination unicast frames loop around a LAN indefinitely. Broadcast storms can saturate all the links with copies of that one single frame, crowding out good frames, as well as significantly impacting end-user PC performance ...

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