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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 path costs for each 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. This makes it more likely for a high-speed port to be selected 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.
The following table lists the default values for path costs.
Table 19-18 Default path cost value
Port speed
10 M bps
100 M bps
1 G bps
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 fastethernet 0/1
(config-if)# spanning-tree cost 2000
Sets MST instances 10, 20, and 30, and sets the path cost of port 0/1 to 2000. This
means the path cost of port 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
(config-if)# exit
Changes the path cost of port 0/1 for MST instance 20 to 500. Instances other than
instance 20 run at 2000.
19 Spanning Tree Protocol
Default path cost value
2000000
200000
20000
333
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