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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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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(conf-if-te-5/41)#do show spanning-tree pvst vlan 2
                                 VLAN 2
                                 Root Identifier has priority 32768, Address 001e.c9f1.00f3
                                 Root Bridge hello time 2, max age 20, forward delay 15
                                 Bridge Identifier has priority 32768, Address 001e.c9f1.00f3
                                 Configured hello time 2, max age 20, forward delay 15
                                 Bpdu filter disabled globally
                                 We are the root of VLAN 2
                                 Current root has priority 32768, Address 001e.c9f1.00f3
                                 Number of topology changes 2, last change occured 00:14:39 ago on Po 23
                                 
                                 Port 24 (Port-channel 23) is designated Forwarding
                                 Port path cost 1600, Port priority 128, Port Identifier 128.24
                                 Designated root has priority 32768, address 001e.c9f1.00:f3
                                 Designated bridge has priority 32768, address 001e.c9f1.00:f3
                                 Designated port id is 128.24 , designated path cost 0
                                 Number of transitions to forwarding state 1
                                 BPDU sent 449, received 0
                                 The port is not in the Edge port mode, bpdu filter is disabled
                                 
                                 Port 450 (TenGigabitEthernet 5/41) is disabled Discarding
                                 Port path cost 2000, Port priority 128, Port Identifier 128.450
                                 Designated root has priority 32768, address 001e.c9f1.00:f3
                                 Designated bridge has priority 32768, address 001e.c9f1.00:f3
                                 Designated port id is 128.450 , designated path cost 0
                                 Number of transitions to forwarding state 0
                                 BPDU sent 0, received 0
                                 The port is not in the Edge port mode, bpdu filter is disabled
                                 
                                 Port 459 (TenGigabitEthernet 5/50) is designated Forwarding
                                 Port path cost 2000, Port priority 128, Port Identifier 128.459
                                 Designated root has priority 32768, address 001e.c9f1.00:f3
                                 Designated bridge has priority 32768, address 001e.c9f1.00:f3
                                 Designated port id is 128.459 , designated path cost 0
                                 Number of transitions to forwarding state 1
                                 BPDU sent 457, received 0
                                 The port is not in the Edge port mode, bpdu filter is disable

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