3Com 3C16476BS-US User Guide - Page 41

Using the System Tools 41 CAUTION: Do not interrupt power to the Switch during the upgrade procedure. If you do, the software may be corrupted and the Switch may not start up properly afterwards. Configuring the Spanning Tree This administrative tool supports the configuration of the Switch to forward, or block and discard 802.1D spanning tree BPDU packets. Spanning tree is a bridge-based system for providing fault tolerance on networks and can be used to detect and disable network loops. The spanning tree ensures that the optimal path is maintained between spanning tree-compliant networked devices by: ■ Disabling redundant paths when the main paths are operational. ■ Enabling redundant paths if the main paths fail. Spanning tree uses a distributed algorithm to select a bridging device that serves as the root of the spanning tree network. The bridging device, known as the Root Bridge, generates bridge protocol data units (BPDUs) on all ports at a regular interval known as the Hello Time. All other spanning tree-compliant devices on the network have a designated Root Port. This is the Port nearest the Root Bridge and it is used for receiving the BPDUs initiated by the Root Bridge. If a bridge does not get a Hello BPDU after a predetermined interval, the bridge assumes that the link to the Root Bridge is down. This bridge will then initiate negotiations with other bridges to reconfigure the network to reestablish a valid network topology. After all the bridges on the network have determined the configuration of their ports, each bridge only forwards traffic between the Root Port and the ports that are the Designated Bridge Ports for each network segment. All other ports are blocked, which means that they are prevented from forwarding traffic. Figure 27 Spanning Tree Screen ■ Forward - Spanning tree BPDU packets received on a port are forwarded to all other ports ■ Block and Discard - Spanning tree BPDUs received on a port are dropped. The Switch does not forward BPDUs to other ports. The Switch does not participate as a bridge node in the spanning tree, it can only be configured to forward or block spanning tree BPDUs. If the Switch is connected to other bridging devices, such as switches, that are part of the spanning tree network, set the Switch to "Forward." If the Switch is connected only to workstations, it can be set to "Block and Discard." 802.1p Prioritization The Switch has priority queuing enabled, which means all packets that are received are examined to see if they have been priority encoded. If a packet has