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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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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 1. Load Balancing with PVST+
Illustration of 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.

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
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
Current root has priority 4096, Address 0001.e80d.b6d6
Number of topology changes 5, last change occurred 00:34:37 ago on Te 1/32

Port 375 (TenGigabitEthernet 1/22) is designated Forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.375
Designated root has priority 4096, address 0001.e80d.b6:d6
Designated bridge has priority 4096, address 0001.e80d.b6:d6
Designated port id is 128.375 , designated path cost 0
Number of transitions to forwarding state 2
BPDU sent 1159, received 632
The port is not in the Edge port mode

Port 385 (TenGigabitEthernet 1/32) is designated Forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.385
Designated root has priority 4096, address 0001.e80d.b6:d6
Designated bridge has priority 4096, address 0001.e80d.b6:d6
Designated port id is 128.385 , designated path cost 0

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