3Com 2928 User Guide - Page 208

1 MSTP Configuration Overview As a Layer 2 management protocol, the Spanning Tree Protocol (STP) eliminates Layer 2 loops by selectively blocking redundant links in a network, and in the mean time, allows for link redundancy. Like many other protocols, STP evolves as the network grows. The later versions of STP are Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP). This chapter describes the characteristics of STP, RSTP, and MSTP and the relationship among them. Introduction to STP STP was developed based on the 802.1d standard of IEEE to eliminate loops at the data link layer in a local area network (LAN). Devices running this protocol detect loops in the network by exchanging information with one another and eliminate loops by selectively blocking certain ports to prune the loop structure into a loop-free tree structure. This avoids proliferation and infinite cycling of packets that would occur in a loop network and prevents decreased performance of network devices caused by duplicate packets received. In the narrow sense, STP refers to the IEEE 802.1d STP; in the broad sense, STP refers to the IEEE 802.1d STP and various enhanced spanning tree protocols derived from that protocol. Protocol Packets of STP STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol packets. STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient information for the network devices to complete spanning tree calculation. In STP, BPDUs come in two types: z Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree topology. z Topology change notification (TCN) BPDUs, used for notifying the concerned devices of network topology changes, if any. Basic Concepts in STP Root bridge A tree network must have a root; hence the concept of root bridge was introduced in STP. There is one and only one root bridge in the entire network, and the root bridge can change along with changes of the network topology. Therefore, the root bridge is not fixed. Upon initialization of a network, each device generates and sends out BPDUs periodically with itself as the root bridge; after network convergence, only the root bridge generates and sends out configuration BPDUs at a certain interval, and the other devices just forward the BPDUs. 1-1