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Influencing 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 different forwarding
topology is generated for each VLAN. This behavior demonstrates how you can use PVST+ to achieve load balancing.
Figure 109. Load Balancing with PVST+
The bridge with the bridge value for bridge priority is elected root. Because all bridges use the default priority (until configured
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.
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
Root Identifier has priority 4096, Address 0001.e80d.b6d6
Root Bridge hello time 2, max age 20, forward delay 15
Bridge Identifier has priority 4096, Address 0001.e80d.b6d6
Configured hello time 2, max age 20, forward delay 15
We are the root of VLAN 100
Per-VLAN Spanning Tree Plus (PVST+)
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