Spanning Tree Protocol (STP)

Enabling Spanning Tree Protocol


NOTE: If more VLANs are defined in the VLAN Trunking Protocol (VTP) than there are spanning-tree instances, you can only have STP on 64 VLANs. If you have more than 128 VLANs, it is recommended that you use Multiple STP.


Configuring the Root Switch



Configuring a Secondary Root Switch


Configuring Port Priority


Configuring the Path Cost


Configuring the Switch Priority of a VLAN


NOTE: With the priority keyword, the range is 0 to 61440 in increments of 4096. The default is 32768. The lower the priority, the more likely the switch will be chosen as the root switch.

Only the following numbers can be used as a priority value:


CAUTION: Cisco recommends caution when using this command. Cisco further recommends that the spanning-tree vlan x root primary or the spanning-tree vlan x root secondary command be used instead to modify the switch priority.

Configuring STP Timers


NOTE: For the hello-time command, the range is 1 to 10 seconds. The default is 2 seconds.
For the forward-time command, the range is 4 to 30 seconds. The default is 15 seconds.
For the max-age command, the range is 6 to 40 seconds. The default is 20 seconds.

CAUTION: Cisco recommends caution when using this command. Cisco further recommends that the spanning-tree vlan x root primary or the spanning-tree vlan x root secondary command be used instead to modify the switch timers.


Verifying STP


Optional STP Configurations

Although the following commands are not mandatory for STP to work, you might find these helpful to fine-tune your network.

PortFast


BPDU Guard




Changing the Spanning-Tree Mode

Different types of spanning tree can be configured on a Cisco switch. The options vary according to the platform:

• Per-VLAN Spanning Tree (PVST)—There is one instance of spanning tree for each VLAN. This is a Cisco proprietary protocol.

• Per-VLAN Spanning Tree Plus (PVST+)—Also Cisco proprietary. Has added extensions to the PVST protocol.

• Rapid PVST+—This mode is the same as PVST+ except that it uses a rapid convergence based on the 802.1w standard.

• Multiple Spanning Tree Protocol (MSTP)—IEEE 802.1s. Extends the 802.1w Rapid Spanning Tree (RST) algorithm to multiple spanning trees. Multiple VLANs can map to a single instance of RST. You cannot run MSTP and PVST at the same time.

Extended System ID


Enabling Rapid Spanning Tree


Troubleshooting Spanning Tree