14 MSTP CONFIGURATIONSTP Overview Functions of STPSpanning tree protocol (STP) is a protocol conforming to IEEE 802.1d. It aims toeliminate loops on data link layer in a local area network (LAN). Devices runningthis protocol detect loops in the network by exchanging packets with one anotherand eliminate the loops detected by blocking specific ports until the network ispruned into one with tree topology. As a network with tree topology is loop-free,it prevents packets in it from being duplicated and forwarded endlessly andprevents device performance degradation.Currently, in addition to the protocol conforming to IEEE 802.1d, STP also refers tothe protocols based on IEEE 802.1d, such as RSTP, and MSTP.Protocol packets of STPSTP uses bridge protocol data units (BPDUs), also known as configurationmessages, as its protocol packets.STP identifies the network topology by transmitting BPDUs between STP compliantnetwork devices. BPDUs contain sufficient information for the network devices tocomplete the spanning tree calculation.In STP, BPDUs come in two types:■ Configuration BPDUs, used to calculate spanning trees and maintain thespanning tree topology.■ Topology change notification (TCN) BPDUs, used to notify concerned devices ofnetwork topology changes, if any.Basic concepts in STP1 Root bridgeA tree network must have a root; hence the concept of "root bridge" has beenintroduced in STP.There is one and only one root bridge in the entire network, and the root bridgecan change alone with changes of the network topology. Therefore, the rootbridge is not fixed.Upon network convergence, the root bridge generates and sends outconfiguration BPDUs periodically. Other devices just forward the configurationBPDUs received. This mechanism ensures the topological stability.2 Root portOn a non-root bridge device, the root port is the port with the lowest path cost tothe root bridge. The root port is used for communicating with the root bridge. A
