IEEE 802.1w

The Spanning Tree Protocol (STP) is a that builds a logical loop-free 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 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.

As the name suggests, STP creates a spanning tree 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.

STP was originally standardized as IEEE 802.1D, but the functionality, spanning tree, rapid spanning tree and previously specified in 802.1D, 802.1w and 802.1s respectively has been incorporated into IEEE 802.1Q-2014

A local area network (LAN) can be depicted as a graph whose nodes are bridges and LAN segments (or cables), and whose edges are the interfaces connecting the bridges to the segments. To break loops in the LAN while maintaining access to all LAN segments, the bridges collectively compute a spanning tree. The spanning tree that the bridges compute using the Spanning Tree Protocol can be determined using the following rules. The example network at the right, below, will be used to illustrate the rules.

Select a root bridge. The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a configurable priority number and a MAC address; the bridge ID is the concatenation of the bridge priority and the MAC address (E.g., the ID of a bridge with priority 32768 and mac 0200.0000.1111 is 32768.0200.0000.1111). The bridge priority default is 32768 and can only be configured in multiples of 4096. When comparing two bridge IDs, the priority portions are compared first and the MAC addresses are compared only if the priorities are equal. The switch with the lowest priority of all the switches will be the root; if there is a tie, then the switch with the lowest priority and lowest MAC address will be the root. For example, if switches A (MAC=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 32768 then switch A will be selected as the root bridge. If the network administrators would like switch B to become the root bridge, they must set its priority to be less than 32768.

...
Wikipedia