Spanning Tree Protocol

Spanning-Tree Verification

Cisco IOS Software Release 12.0	2900#show spanning-tree brief	Displays the spanning-tree table of the switch
Cisco IOS Software Release 12.1	Switch#show spanning-tree	Displays the spanning-tree table of the switch
	2950#show spanning-tree int fa 0/17	Displays spanning-tree info for port fa 0/17
	2950#show spanning-tree vlan x	Displays spanning-tree info for the specific VLAN
	2950#debug spanning-tree {all}	Displays all informational messages about changes in the spanning-tree topology

Change Spanning-Tree Priority of a Switch

Cisco IOS Software Release 12.0	Switch(config)#spanning-tree priority 1	Number can be from 165535. Lower number means better chance of being elected the root bridge. The default is 32768
Cisco IOS Software Release 12.1	Switch(config)#spanning-tree vlan 1 priority 1	Number can be from 165535. Lower number means better chance of being elected the root bridge. The default is 32768
	Switch#spanning-tree vlan x root	Changes this switch to the root switch for VLAN x by lowering its priority to either 24576 or 100 less than current root bridge, whichever number is lower

Changing the Cost of Spanning Tree on an Interface

Switch#config t
Switch(config)#int fa 0/1
Switch(config-if)#spanning-tree cost x	Sets spanning-tree cost to specified value of x

Changing the State of Spanning Tree on an Interface

Switch(config)#int fa 0/1
Switch(config-if)#spanning-tree portfast	See the Caution that follows

Caution:

The command spanning-tree portfast will force a port to move directly to the Forwarding state, without having to transition through the Blocking, Listening, and Learning states.

This can save up to 50 seconds of wait time. This is an excellent command on access ports that will never be hooked up to another switch. Examples where this would be useful are on ports connected to computers or other end devices, such as printers, servers, and so on. You must exercise caution with this command, however, because if a switch port is plugged into another switch and the portfast command is enabled, then spanning tree will be defeated and a potential switching loop might be created.

Spanning Tree Portfast BPDU Guard Command

Switch#config t
Switch(config)#spanning-tree portfast bpduguard	Enables BPDU Guard for all interfaces
Switch(config)#errdisable recovery cause bpduguard	Allows port to re-enable itself by setting a recovery timer
Switch(config)#errdisable recovery interval 400	Sets recovery timer to 400 seconds. Default is 300 seconds
Switch(config)#show spanning-tree summary totals	Verifies if BPDU Guard is enabled or disabled
Switch#show errdisable recovery	Displays errdisable recovery timer information

Note:

There is a feature that has been added to Catalyst IOS called Spanning Tree Portfast BPDU Guard. This feature has been created to maintain stability within the active STP topology. Devices that are attached to ports with Portfast enabled are not allowed to take part in the spanning-tree process. If a port that has Portfast enabled receives a BPDU packet, that port is disabled by the switch. This is done by transitioning the port into the errdisable state. An error message will be sent to the console. Ports disabled by STP BPDU Guard remain in the disabled state unless it is manually enabled with the no shut command or configured to re-enable itself automatically.