By default, spanning tree is enabled on all Cisco switches. Creating an additional VLAN automatically runs another instance of the spanning tree protocol. Spanning tree configuration consists of the following tasks:
The following table lists commands you would use to configure spanning tree:
Command |
Function |
Switch(config)#spanning-tree mode pvst|rapid-pvst|mst |
Sets the spanning tree mode. |
Switch(config)#spanning-tree vlan <1-4094> root primary |
Forces the switch to be the root of the spanning tree.
|
Switch(config)#spanning-tree vlan <1-4094> priority <0-61440> |
Manually sets the bridge priority number.
|
Switch(config-if)#spanning-tree portfast |
Enables PortFast on the interface. When the PortFast feature is enabled, the interface changes directly from a blocking state to a forwarding state without making the intermediate spanning-tree state changes. Note: This command is for an edge-type interface. Configuring PortFast on interfaces connected to hubs, concentrators, switches, and bridges can cause temporary bridging loops. |
Switch(config)#spanning-tree uplinkfast |
Enables UplinkFast to accelerate the choice of a new root port when a link or switch fails or when the spanning tree reconfigures itself. Note: When you configure rapid spanning tree, disable UplinkFast. Similar functionality is built into rapid spanning tree. |
Switch(config)#no spanning-tree vlan <1-4094> |
Disables spanning tree on the selected VLAN. |
Switch#show spanning-tree |
Show spanning tree configuration information including the following:
To verify that spanning tree is working, look for an entry similar to the following for each VLAN: Spanning tree enabled protocol ieee |
Switch#show spanning-tree vlan <1-4094> root |
Show information about the root bridge for a specific VLAN. Information shown includes:
|
Switch#show spanning-tree vlan <1-4094> bridge |
Show spanning tree configuration information about the local switch for the specified VLAN. Information includes the local bridge ID, including the priority and MAC address. |
Examples
The following commands set the bridge priority for a VLAN and enables PortFast on two ports:
Switch(config)#spanning-tree vlan 20 priority 4096
Switch(config)#int fa0/12
Switch(config-if)#spanning-tree portfast
Switch(config-if)#int fa0/13
Switch(config-if)#spanning-tree portfast
The following commands enable Rapid PVST+ for the switch, sets the bridge priority to a higher value than the default, and disables UplinkFast:
Switch(config)#spanning-tree mode rapid-pvst
Switch(config)#spanning-tree vlan 1 priority 36864
Switch(config)#no spanning-tree uplinkfast
Note: When switching from PVST+ to Rapid PVST+, PortFast designations can still be used.
Credit: Testout 640-802 CCNA Notes
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