The Spanning Tree Protocol (STP) prevents network failures by establishing a loop-free topology for layer two bridging devices. Bridges send Bridge Protocol Data Units (BPDUs) to each other to share information and recalculate the topology when changes occur. The protocol was developed by DEC and incorporated into the IEEE 802.1D standard. Rapid Spanning Tree Protocol (RSTP) replaced STP as the standard protocol for loopless bridging, and Multiple Spanning Tree Protocol (MSTP) allows separate instances of STP to coexist in multiple VLAN configurations.
Network devices that serve as open systems interconnect model (OSI) layer two bridging devices, including switches, are often redundantly connected to each other to help prevent network failures. Spanning tree protocol (STP) is a set of communication rules, methods, and functions that are implemented in bridge software that prevent data link layer frames from looping within a redundant bridge topology. Spanning tree protocol is a data link layer protocol that establishes a single, low-cost path for data transfer from any connected network endpoint to another.
Bridges using the spanning tree protocol send special messages called bridge protocol data units (BPDUs) to each other in order to share information and inform each other about network changes. Most bridges participate in the spanning tree process, but network administrators might designate some bridges as pass-through-only devices. All bridging devices actively using the spanning tree protocol swap BPDUs in order to establish and maintain a minimal loop-free spanning tree topology.
STP starts recalculating a new spanning tree topology whenever bridges are added or removed from the network, when paths within the network fail, when Unidirectional Link Discovery (UDLD) detects a broken fiber, or when other topological changes that interrupt the established flow of data. Bridges actively using the spanning tree protocol begin topological reconfiguration by first agreeing and designating one of the bridging devices as the root node for the new spanning tree. They then collaborate by sending each other BPDUs containing current information and path costs to compute a new spanning tree that has the lowest cost for the topology. As soon as the bridges agree on a new topology, bridge ports connected to redundant segments that are not part of the new spanning tree and are not part of a designated etherchannel are temporarily blocked to prevent forwarding of data traffic. Blocking these unused redundant paths creates a loop-free topology.
The spanning tree protocol was initially developed and implemented by Digital Equipment Corporation (DEC). It was incorporated into the Institute of Electrical and Electronics Engineers (IEEE) 802.1D standard shortly thereafter. The IEEE 802.1w proposal introduced a newer and faster version of the spanning tree protocol called the rapid spanning tree protocol (RSTP). RSTP replaced STP as the standard protocol for loopless bridging and was designated the protocol of choice in the revised IEEE 802.1D-2004 standard. STP was further refined with the release of the Multiple Spanning Tree Protocol (MSTP) as detailed in IEEE 802.1Q-2005, which allows separate instances of STP to coexist in multiple virtual local area network (VLAN) configurations.
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