The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails. As the name suggests, STP creates a spanning tree that characterizes the relationship of nodes within a network of connected layer-2 bridges, and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. STP is based on an algorithm that was invented by Radia Perlman while she was working for Digital Equipment Corporation. In 2001, the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. RSTP provides significantly faster recovery in response to network changes or failures, introducing new convergence behaviors and bridge port roles to do this. RSTP was designed to be backwards-compatible with standard STP. STP was originally standardized as IEEE 802.1D but the functionality of spanning tree (802.1D), rapid spanning tree (802.1w), and Multiple Spanning Tree Protocol (802.1s) has since been incorporated into IEEE 802.1Q-2014. While STP is still in use today, in many modern networks its primary use is as a loop-protection mechanism rather than a fault tolerance mechanism. Link aggregation protocols such as LACP bond multiple links to provide link-level fault tolerance while simultaneously increasing overall link capacity. However, implementing switch-level redundancy requires multi-chassis link aggregation, of which only proprietary variants exist. Shortest Path Bridging combines both loop-free forwarding and link aggregation, but hasn't reached STP's or LACP's popularity yet, as of 2026.
Protocol operation
The need for the Spanning Tree Protocol (STP) arose because switches in local area networks (LANs) are often interconnected using redundant links to improve resilience should one connection fail. However, this connection configuration creates a switching loop resulting in broadcast radiations and MAC table instability. If redundant links are used to connect switches, then switching loops need to be avoided. To avoid the problems associated with redundant links in a switched LAN, STP is implemented on switches to monitor the network topology. Every link between switches, and in particular redundant links, is catalogued. The spanning-tree algorithm then blocks forwarding on redundant links by setting up one preferred link between switches in the LAN. This preferred link is used for all Ethernet frames unless it fails, in which case a non-preferred redundant link is enabled. When implemented in a network, STP designates one layer-2 switch as root bridge. All switches then select their best connection towards the root bridge for forwarding and block other redundant links. All switches constantly communicate with their neighbors in the LAN using § Bridge protocol data units (BPDUs). Provided there is more than one link between two switches, the STP root bridge calculates the cost of each path based on bandwidth. STP will select the path with the lowest cost, that is, the highest bandwidth, as the preferred link. STP will enable this preferred link as the only path to be used for Ethernet frames between the two switches, and disable all other possible links by designating the switch ports that connect the preferred path as root port. After STP-enabled switches in a LAN have elected the root bridge, all non-root bridges assign one of their ports as a root port. This is either the port that connects the switch to the root bridge, or if there are several paths, the port with the preferred path as calculated by the root bridge. Because not all switches are directly connected to the root bridge, they communicate amongst each other using STP BPDUs. Each switch adds the cost of its own path to the cost received from the neighboring switches to determine the total cost of a given path to the root bridge. Once the cost of all possible paths to the root bridge has been added up, each switch assigns a port as a root port that connects to the path with the lowest cost, or highest bandwidth, that will eventually lead to the root bridge.
Path cost
The STP path cost default was originally calculated by the formula 1 Gbit/s/bandwidth. When faster speeds became available, the default values were adjusted as otherwise speeds above 1 Gbit/s would have been indistinguishable by STP. Its successor RSTP uses a similar formula with a larger numerator: 20 Tbit/s/bandwidth. These formulas lead to the sample values in the table.
Port states All switch ports in the LAN where STP is enabled are categorized.
… excerpt ends here. Continue reading the full article.
