Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-1Chapter 1 MSTP Configuration1.1 STP OverviewI. 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 running thisprotocol detect loops in the network by exchanging packets with one another andeliminate the loops detected by blocking specific ports until the network is pruned intoone with tree topology. As a network with tree topology is loop-free, it prevents packetsin 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 theprotocols based on IEEE 802.1d, such as RSTP, and MSTP.II. Protocol packets of STPSTP uses bridge protocol data units (BPDUs), also known as configuration messages,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:z Configuration BPDUs, used to calculate spanning trees and maintain the spanningtree topology.z 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 bridge canchange alone with changes of the network topology. Therefore, the root bridge is notfixed.Upon network convergence, the root bridge generates and sends out configurationBPDUs periodically. Other devices just forward the configuration BPDUs received. Thismechanism ensures the topological stability.
