In today’s internet-dependant world, network redundancy is a must. Network integrity often demands multiple bridges within networks and to the internet to prevent catastrophic downtime caused by a single edge failure (ie cut cable or power loss to a switch). The problem encountered with computer networks is that they were not designed to handle loops within the network. The introduction of a loop into a computer network can cause crippling flooding of the network as packets circle the loop indefinitely. One option would be to allow for manual enabling/disabling of bridges within a network in the event of a failure. Though feasible, this would require constant monitoring and quick response time. Enter the Spanning Tree Protocol (STP), IEEE 802.1D.

The Spanning Tree Protocol allows communication of the state of network topology as well as control commands over the state of network bridges. For initiation of the spanning tree, a root node is first selected by a user-settable priority; this is usually the main host gateway. When the root node is established, each node determines the least-cost pathway to the root node. The user can also set edge “weights” in configuration, which usually correlated to bandwidth. Using this data, in a form of Prim’s Algorithm, the minimum spanning tree is determined. Commands are then sent to disable potential bridge loops within the network. Take for example the accompanying figure, where green lines represent the determined minimum spanning tree and red lines are disabled bridges with the root node in blue and all edges having equal weight. Nodes then regularly send “Hello”s to each other so that when communication terminates between two nodes, a bridge can be re-enabled to restore communication, eliminating the need for human intervention.

Though technologies like STP increase the reliability of computer networks, their need speaks to intrinsic inadequacies in now-ancient networking technologies. STP, with base configuration, still has a 6 second recovery time, which could me severe loss for trading and banking companies. Luckily, innovations in networking, including development of wireless mesh networking technologies, are clearing the way for new protocols that will soon further enhance the reliability and accessibility of the internet.

Reference Article: STP: Spanning Tree Protocol in IEEE 802.1D