Which command is used to modify an MST instance?

The correct command to modify an MST (Multiple Spanning Tree) instance is one that places the device into global configuration mode, allowing various configurations, including MST adjustments, to be made. Entering global configuration mode is necessary for any changes to the spanning tree configuration to take effect.

When you use the command to enter global configuration mode, you can then specify the MST instances and configure parameters such as bridge priority or the number of VLANs associated with the instance. This step is essential because various network configurations are managed from this mode, making it the gateway to modifying MST settings and other configurations as needed.

Other commands listed are not specifically correct for directly modifying an MST instance. For instance, while "config mst" might seem like a relevant option, it is not a recognized command in Cisco IOS for entering MST configuration directly. "Spanning-tree modify" does not exist as a command, and "spanning-tree mst update" does not serve to modify an MST instance specifically; rather, it seems to suggest an update operation. Thus, understanding the need to first access configuration mode clarifies why entering into global configuration is the appropriate step for making these modifications.