3Com 7600 manual Spanning Tree Protocol

Spanning Tree Protocol 11-9

transparent bridges the bridged LAN can contain paths on which packets may loop and multiply, lowering its effective overall bandwidth.

Figure 11-4Packet looping through redundant bridges

To solve this problem, the IEEE 802.1d Bridging Standard describes the Spanning Tree Protocol (STP), an algorithm which seeks to establish a unique path between each pair of LANs in the bridged network by selectively blocking redundant bridge ports. The resulting loop-free tree spans the network: each LAN can access any other LAN. The STP continuously monitors the tree and, in the event of a failure, acts to establish a new spanning tree by reactivating blocked ports.

The STP is executed collectively by the bridges themselves by means of special messages called Configuration Bridge Protocol Data Units (CBPDUs) continuously sent between them. Using these messages, the bridges inform the other bridges of their unique bridge-ID, and they calculate the shortest (lowest cost) path from themselves to other locally-connected bridges. Using this information, the STP determines the following:

Root Bridge The root-bridgeis selected among all bridges on the LANs as the one with the lowest bridge ID. The network manager can influence the algorithm’s choice of root bridge through the management parameter bridge-priority. The bridge-priority is appended as the most significant part of a bridge ID; the lower its value, the more likely the bridge is to become the root.

Root Port Once the root bridge has been determined, each bridge selects the port with the shortest path from it to the root bridge to be the root-port.