Final answer:
To modify the default RIPv2 behavior of automatic summarization, use the no auto-summary command.
Step-by-step explanation:
To modify the default RIPv2 behavior of automatic summarization, you would need to disable summarization with the no auto-summary command. By default, RIPv2 automatically summarizes network prefixes at their classful network boundaries. However, using the no auto-summary command disables this behavior, allowing more specific network prefixes to be advertised.