Final answer:
Using the "no auto-summary" command disables automatic summarization in EIGRP, which prevents the router from summarizing routes at network boundaries, ensuring more precise and potentially more efficient routing.
Step-by-step explanation:
After using the "no auto-summary" command in a router configuration, automatic summarization is disabled in the Enhanced Interior Gateway Routing Protocol (EIGRP).
This command prevents the router from automatically summarizing routes at network boundaries, which can be essential in networks that do not adhere to classful addressing. By default, EIGRP summarizes routes to their classful boundaries. When auto-summarization is disabled, EIGRP instead advertises the full subnet mask along with the route, providing more precise routing information and preventing issues such as routing loops or suboptimal routing paths that can occur due to incorrect summarization.