After you change the IP address on an IOS XR router, you cannot ping the new address. Which step did you forget to complete?

Cisco IOS XR uses a two-stage configuration model. When you enter configuration commands, they are added to a target or "candidate" configuration, which is not active. The commit command validates this candidate configuration and applies it to the running configuration, making the changes take effect. Without committing, the new IP address is not active on the interface, and thus, it will not respond to pings. This process ensures configuration changes are applied atomically and can be validated before activation.

B. Rolling back the configuration would revert the change, not apply it. This is the opposite of the desired outcome.
C. Merging a configuration is a method to load changes, but the commit command is still the final step required to activate them.
D. Saving the running configuration (copy running-config startup-config in classic IOS) is not the command used in IOS XR to activate changes.

1. Cisco IOS XR System Management Configuration Guide, Release 7.3.x, "Implementing Configuration Management on Cisco IOS XR Software" chapter, "Configuration Fundamentals" section. It states, "Configuration changes are not applied to the system until they are committed. When a configuration is committed, it becomes part of the running configuration."

2. Cisco IOS XR Getting Started Guide for the Cisco CRS Router, "Basic Router Configuration" chapter, "Committing the Configuration" section. It details the necessity of the commit command to make any configuration changes active.