1-11 MSTP Configurationz The term switch used throughout this chapter refers to a switching device in a generic sense or theswitching engine of a unified switch in the WX3000 series.z The sample output information in this manual was created on the WX3024. The output informationon your device may vary.STP OverviewSTP OverviewFunctions of STPSpanning tree protocol (STP) is a protocol conforming to IEEE 802.1d. It aims to eliminate loops on datalink layer in a local area network (LAN). Devices running this protocol detect loops in the network byexchanging packets with one another and eliminate the loops detected by blocking specific ports untilthe network is pruned into one with tree topology. As a network with tree topology is loop-free, itprevents packets in it from being duplicated and forwarded endlessly and prevents device performancedegradation.Currently, in addition to the protocol conforming to IEEE 802.1d, STP also refers to the protocols basedon IEEE 802.1d, such as RSTP, and MSTP.Protocol packets of STPSTP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocolpackets.STP identifies the network topology by transmitting BPDUs between STP compliant network devices.BPDUs contain sufficient information for the network devices to complete the spanning tree calculation.In STP, BPDUs come in two types:z Configuration BPDUs, used to calculate spanning trees and maintain the spanning tree topology.z Topology change notification (TCN) BPDUs, used to notify concerned devices of network topologychanges, if any.Basic concepts in STP1) Root bridgeA tree network must have a root; hence the concept of “root bridge” has been introduced in STP.There is one and only one root bridge in the entire network, and the root bridge can change alone withchanges of the network topology. Therefore, the root bridge is not fixed.
