For questions about Spanning Tree Protocol (STP), a common protocol to prevent bridge loops at OSI layer-2.
The Spanning Tree Protocol (STP) is a network protocol that builds a logical loop-free topology for bridged OSI layer-2 networks. The basic function of STP is to prevent bridge loops, broadcast radiation and SAT instabilities that result from them.
Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails. This is done without the danger of bridge loops, or the need for manual enabling or disabling of these backup links.
Originally defined by (now obsolete) IEEE 802.1D-1990, the functionality was significally improved by the Rapid Spanning Tree Protocol (RSTP) in IEEE 802.1w-2001 and the VLAN-aware and instance-capable Multiple Spanning Tree Protocol (MSTP) in IEEE 802.1s-2002, now all part of IEEE 802.1Q.