26.8.3 Multiple spanning tree topology settings

(1) Setting Bridge Priority Per Instance

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 in case 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.

The bridge priority for Multiple Spanning Tree is set for each instance. When values are changed for each instance, load balancing (building different topologies) can be performed per instance.

Command examples

1. (config)# spanning-tree mst 0 root priority 4096

   (config)# spanning-tree mst 20 root priority 61440

   Sets the bridge priority of the CIST (instance 0) to 4096, and the bridge priority of instance 20 to 61440.

(2) Setting the Path Cost per Instance

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)# spanning-tree mst configuration

   (config-mst)# instance 10 vlans 100-150

   (config-mst)# instance 20 vlans 200-250

   (config-mst)# instance 30 vlans 300-350

   (config-mst)# exit

   (config)# interface gigabitethernet 1/0/1

   (config-if)# spanning-tree cost 2000

   Sets MST instances 10, 20, and 30, and sets the path cost of port 1/0/1 to 2000. This means the path cost of port 1/0/1 is 2000 for the CIST (instance 0) and MST instances 10, 20, and 30.

2. (config-if)# spanning-tree mst 20 cost 500

   Changes the path cost of port 1/0/1 for MST instance 20 to 500. Instances other than instance 20 run at 2000.

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 Per Instance

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 mst 20 port-priority 144

   Sets the port priority of port 1/0/1 for instance 20 to 144. For port 1/0/1, only instance 20 has a port priority of 144, with other instances running at 64.