Reference Guide

718 | Per-VLAN Spanning Tree Plus (PVST+)
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Figure 38-2. Display the PVST+ Configuration
Influencing PVST+ Root Selection
In Figure 38-1, all VLANs use the same forwarding topology because R2 is elected the root, and all
Ten-Gigabit Ethernet ports have the same cost. Figure 38-3 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 38-3. Load Balancing with PVST+
The bridge with the bridge value for bridge priority is elected root. Since all bridges use the default priority
(until configured otherwise), lowest MAC address is used as a tie-breaker. Assign bridges a low
non-default value for bridge priority to increase the likelihood that it will be selected as the STP root.
Display the PVST+ forwarding topology by entering the command
show spanning-tree pvst [vlan vlan-id]
from EXEC Privilege mode, as shown in Figure 38-4.
Task Command Syntax Command Mode
Assign a bridge priority.
VLAN range: 1 to 4094.
Priority range: 0 to 61440
Default priority: 32768
vlan vlan-range bridge-priority value
PROTOCOL PVST
FTOS(conf-pvst)#show config verbose
!
protocol spanning-tree pvst
no disable
vlan 100 bridge-priority 4096
R2
R3
R1
2/32
1/22
1/32
2/12
3/22
3/12
STI 3 root
STI 3: VLAN 300
STI 1: VLAN 100
STI 2: VLAN 200STI 2: VLAN 200
STI 1 root
Forwarding
XX
XX
XX
STI 2 root
vlan 200 bridge-priority 4096
vlan 100 bridge-priority 4096
vlan 300 bridge-priority 4096
Blocking