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19.7.3 Configuring topologies for Single Spanning Tree
(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 single priority 4096
Sets the bridge priority for Single Spanning Tree to 4096.
(2) Setting path costs
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.
Path cost values consist of two types, short (16-bit values) and long (32-bit values),
either of which must be used over an entire topology. By default, short (16-bit value)
types are used for operation. Automatic settings based on Ethernet interface speed
differ depending on whether short (16-bit value) or long (32-bit value) types are set.
The following table lists the default values for path costs.
Table 19-14 Default path cost value
Port speed
10 M bps
100 M bps
1 G bps
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.
Default path cost value
short (16-bit value)
100
19
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19 Spanning Tree Protocol
long (32-bit value)
2000000
200000
20000
319
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