3Com 3C16476BS-US - Baseline 2250 Plus Switch User Manual

Contents

Using the System Tools 41CAUTION: Do not interrupt power to the Switchduring the upgrade procedure. If you do, the soft-ware may be corrupted and the Switch may not startup properly afterwards.Configuring the Spanning TreeThis administrative tool supports the configuration ofthe Switch to forward, or block and discard 802.1Dspanning tree BPDU packets.Spanning tree is a bridge-based system for providingfault tolerance on networks and can be used todetect and disable network loops. The spanning treeensures that the optimal path is maintained betweenspanning tree-compliant networked devices by:■ Disabling redundant paths when the main pathsare operational.■ Enabling redundant paths if the main paths fail.Spanning tree uses a distributed algorithm to select abridging device that serves as the root of the span-ning tree network. The bridging device, known as theRoot Bridge, generates bridge protocol data units(BPDUs) on all ports at a regular interval known as theHello Time. All other spanning tree-compliant deviceson the network have a designated Root Port. This isthe Port nearest the Root Bridge and it is used forreceiving the BPDUs initiated by the Root Bridge. If abridge does not get a Hello BPDU after a predeter-mined interval, the bridge assumes that the link to theRoot Bridge is down. This bridge will then initiatenegotiations with other bridges to reconfigure thenetwork to reestablish a valid network topology.After all the bridges on the network have determinedthe configuration of their ports, each bridge only for-wards traffic between the Root Port and the portsthat are the Designated Bridge Ports for each networksegment. All other ports are blocked, which meansthat they are prevented from forwarding traffic.Figure 27 Spanning Tree Screen■ Forward – Spanning tree BPDU packets received ona port are forwarded to all other ports■ Block and Discard – Spanning tree BPDUs receivedon a port are dropped. The Switch does not for-ward BPDUs to other ports.The Switch does not participate as a bridge node inthe spanning tree, it can only be configured to for-ward or block spanning tree BPDUs. If the Switch isconnected to other bridging devices, such asswitches, that are part of the spanning tree network,set the Switch to "Forward." If the Switch is con-nected only to workstations, it can be set to "Blockand Discard."802.1p PrioritizationThe Switch has priority queuing enabled, whichmeans all packets that are received are examined tosee if they have been priority encoded. If a packet has