How can UDLD be enabled globally on a switch?

Enabling Unidirectional Link Detection (UDLD) globally on a switch is accomplished through the command that activates this feature across all relevant interfaces. The command "udld enable" achieves this goal, allowing the switch to monitor and manage unidirectional link conditions effectively. When UDLD is enabled, the switch can detect and mitigate issues caused by unidirectional links, helping to prevent potential network loops and ensuring reliable communication between devices.

The other options do not provide valid commands for this action. For instance, "udld start" and "enable udld global" are not recognized commands in Cisco IOS for UDLD configuration. Additionally, configuring UDLD solely through VLAN configuration would not enable it globally across all interfaces, which is necessary for comprehensive network monitoring and link management.