Service Manual

Inuencing PVST+ Root Selection
As shown in the previous per-VLAN spanning tree illustration, all VLANs use the same forwarding topology because R2 is elected the root,
and all TenGigabitEthernet ports have the same cost.
The following per-VLAN spanning tree illustration changes the bridge priority of each bridge so that a dierent forwarding topology is
generated for each VLAN. This behavior demonstrates how you can use PVST+ to achieve load balancing.
Figure 101. Load Balancing with PVST+
The bridge with the bridge value for bridge priority is elected root. Because all bridges use the default priority (until congured otherwise),
the lowest MAC address is used as a tie-breaker. To increase the likelihood that a bridge is selected as the STP root, assign bridges a low
non-default value for bridge priority.
To assign a bridge priority, use the following command.
• Assign a bridge priority.
PROTOCOL PVST mode
vlan bridge-priority
The range is from 0 to 61440.
The default is 32768.
Example of the show spanning-tree pvst vlan Command
To display the PVST+ forwarding topology, use the show spanning-tree pvst [vlan vlan-id] command from EXEC Privilege
mode.
Dell_E600(conf)#do show spanning-tree pvst vlan 100
VLAN 100
640
Per-VLAN Spanning Tree Plus (PVST+)