Wikipedia
A switching loop or bridge loop occurs in computer networks when there is more than one layer 2 path between two endpoints (eg multiple connections between two network switches or two ports on the same switch connected to each other) The loop creates broadcast storms as broadcasts and multicasts are forwarded by switches out every port, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network Since the layer-2 header does not include a time to live (TTL) field, if a frame is sent into a looped topology, it can loop forever A physical topology that contains switching or bridge loops is attractive for redundancy reasons, yet a switched network must not have loops
Switching loop - Wikipedia, the free encyclopedia