Cisco Certified Network Associate (CCNA) Practice Exam

Which command forces a switch to become the root bridge for a specific VLAN?

1. spanning-tree vlan VLAN NUMBER root secondary

2. spanning-tree vlan VLAN NUMBER root primary

3. spanning-tree root bridge for VLAN

4. set bridge root VLAN NUMBER

The correct answer is: spanning-tree vlan VLAN NUMBER root primary

The command "spanning-tree vlan VLAN NUMBER root primary" is used to configure a switch to become the root bridge for a specific VLAN. When this command is issued, it increases the bridge priority for that switch to a value that is lower than the default, effectively making it more likely to be elected as the root bridge. In the Spanning Tree Protocol (STP), root bridge election is determined by the bridge priority and MAC address. The lower the bridge priority value, the more likely the switch will become the root bridge. By using the "root primary" command, the administrator ensures that the switch is favored in the election process for the specified VLAN. This focus on a specific VLAN allows network administrators to have fine-grained control over the STP topology, guiding which switch should serve as the root bridge based on the network design and requirements, ultimately improving network performance and reducing potential issues caused by loops. Other options do not correctly represent valid commands for configuring a switch as a root bridge in this context, and thus would not achieve the desired outcome.