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19 Spanning Tree Protocol
Command examples
1.
(config)# interface fastethernet 0/1
(config-if)# spanning-tree cost 100
(config-if)# exit
Sets the path cost of port 0/1 to 100.
2.
(config)# spanning-tree pathcost method long
(config)# interface fastethernet 0/1
(config-if)# spanning-tree vlan 10 cost 200000
(config-if)# exit
Sets long (32-bit value) path costs to be used, and then changes port 0/1 for VLAN
10 to have a cost value of 200000. The path cost is 200000 on port 0/1 for only VLAN
10, with other VLANs running at 100.
Notes
When link aggregation is used, the default value for the path costs of a channel
group is not the total of all ports in the channel group, but the speed of a single port.
(3) Setting port priority
The port priority is set to determine which port is used when a Spanning Tree Protocol is
used to make connections between two switches redundant, and the path costs are the
same value for both.
Normally, we recommend that you use link aggregation as functionality to make
connections between two switches redundant, but use this functionality when a Spanning
Tree Protocol is needed for redundancy because the partner connected switch does not
support link aggregation.
Points to note
For port priorities, a lower value indicates a higher priority. When redundancy is used
between two switches, the path whose switch is closer to the root bridge and whose
port has a higher priority is used as the communication path. If this parameter is not
set, the port with the lower port number is prioritized.
Command examples
1.
(config)# interface fastethernet 0/1
(config-if)# spanning-tree port-priority 64
(config-if)# exit
Sets the port priority for port 0/1 to 64.
2.
(config)# interface fastethernet 0/1
(config-if)# spanning-tree vlan 10 port-priority 144
(config-if)# exit
Changes the port priority of port 0/1 for VLAN 10 to 144. For port 0/1, only VLAN 10
has a port priority of 144, with other VLANs running at 64.
19.4.4 Configuring PVST+ parameters
Each parameter must be set to satisfy the following relationship: 2 x (forward-time - 1) ≥
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