What command is used to set the STP priority to secondary for a VLAN?

To set the Spanning Tree Protocol (STP) priority to secondary for a VLAN, the appropriate command is "spanning-tree vlan vlan-id root secondary." This command is specifically designed to designate a switch as the secondary root bridge for the specified VLAN. In an STP topology, the root bridge is selected based on the lowest bridge ID, which is derived from the bridge priority and the MAC address of the switch. By using this command, you effectively assign a priority value that is higher than the primary root bridge, thereby positioning this switch as a secondary option only if the primary root bridge fails.

This command is important in creating a more resilient network topology, as having designated primary and secondary root bridges helps in ensuring that there is a backup in case the primary goes offline.

The other commands serve different purposes. The command for "root primary" would set the switch as the primary root bridge, the cost command relates to the cost of the path toward the root bridge, and the port-priority command is used to prioritize specific ports rather than defining the overall bridge priority for the VLAN. Understanding these commands helps in effectively managing and optimizing network operations using STP.