Configuring the WSM using Device Manager

Multiple spanning tree example--determining root cost    

The following process describes how root cost is determined in the Example--four-WSM topology with multiple spanning trees.

• Once WSMs B and D receive the BPDU from the root bridge (WSM A), they modify the BPDU by adding their bridge ID numbers. They then forward the BPDU out their egress ports.
• When WSM C receives BPDUs from both B and D, it can immediately determine from the information in the BPDU that they are both part of the same spanning tree. WSM C thus knows that bridging them together will create a loop in the network. At this point, C makes a decision that one port will be used for forwarding, and the other port will be blocked.
• If the ports are tagged, each port sends out a special BPDU containing the tagged information.
• When determining which port to use for forwarding and which port to block, WSM D uses information in the BPDU, including each bridge priority ID. The lowest root cost is then computed to determine the most efficient path for forwarding. For more information on bridge priority, port priority, and port cost, refer to the Web OS Switch Software 10.0 Command Reference, Part number 212778-A. .

See also:

• Root cost comparison
• Example--four-WSM topology with multiple spanning trees
• About Spanning Tree protocol
• VLANs and spanning tree protocol
• Understanding multiple spanning trees
• Why multiple Spanning Trees?
• Example--four-WSM topology with single spanning tree
• Configuring spanning tree protocol