On an Ethernet switching network, redundant links are used to implement link backup and enhance network reliability. However, the use of redundant links may produce loops, leading to broadcast storms and an unstable MAC address table. As a result, communication on the network may deteriorate or even be interrupted.
To prevent loops, IEEE introduced the Spanning Tree Protocol (STP), which is standardized as IEEE 802.1D. STP prevents loops on a local area network (LAN). Devices running STP exchange STP bridge protocol data units (BPDUs) to discover loops on the network and block appropriate ports. This enables a ring topology to be trimmed into a loop-free tree topology. If an active link fails and a redundant link exists, STP activates the redundant link to ensure network connectivity.
The convergence speed of an STP topology slows as the number of LANs increases. Therefore, IEEE introduced the Rapid Spanning Tree Protocol (RSTP), standardized as 802.1w, in 2001 to improve the network convergence speed.