28.4.3 PVST + topology settings

(1) Setting Bridge Priority

The bridge priority is a parameter for determining the root bridge. When a topology is designed, the highest priority is set for the switch to be used for the root bridge, and the second highest priority is set for the switch to be used next for the root bridge if a fault occurs on the root bridge.

Points to note

For bridge priorities, a lower value indicates a higher priority, and the switch with the lowest set value is the root bridge. Because the root bridge is decided by a bridge ID consisting of the bridge priority and switch MAC address, if this parameter is not set, the switch with the lowest MAC address becomes the root bridge.

Command examples

1. (config)# spanning-tree vlan 10 priority 4096

   Sets the bridge priority for the VLAN 10 PVST+ to 4096.

(2) Set path cost

The path cost is a parameter for determining communication paths. When a Spanning Tree topology is designed, after the bridge priority is determined, the root port of each designated bridge (communication path from the designated bridge to the root bridge) is determined by using this parameter.

Points to note

Path cost values are set for each port of a designated bridge. Small values can be set to make root port selection more likely. If no value is set, different default values are used for each port speed, with faster ports more likely to be chosen for the root port.

Path costs are set to prioritize the use of slow ports over fast ports as paths. No settings are needed for topologies in which fast ports are prioritized.

Command examples

1. (config)# interface gigabitethernet 1/0/1

   (config-if)# spanning-tree cost 100

   (config-if)# exit

   Sets the path cost of port 1/0/1 to 100.

2. (config)# spanning-tree pathcost method long

   (config)# interface gigabitethernet 1/0/1

   (config-if)# spanning-tree vlan 10 cost 200000

   Sets long (32-bit value) path costs to be used, and then changes port 1/0/1 for VLAN 10 to have a cost value of 200000. The path cost is 200000 on port 1/0/1 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) Configuring 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 gigabitethernet 1/0/1

   (config-if)# spanning-tree port-priority 64

   (config-if)# exit

   Sets the port priority for port 1/0/1 to 64.

2. (config)# interface gigabitethernet 1/0/1

   (config-if)# spanning-tree vlan 10 port-priority 144

   Changes the port priority of port 1/0/1 for VLAN 10 to 144. For port 1/0/1, only VLAN 10 has a port priority of 144, with other VLANs running at 64.