Which command is used on a Cisco router to display EIGRP neighbor relationships?
Correct Answer: A
Detailed Explanation: Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol that relies on exchanging information between directly connected routers. One of the key aspects of managing and troubleshooting EIGRP is verifying neighbor relationships—i.e., the routers that have established adjacencies with one another to share routing information. The command “show ip eigrp neighbors” is used to display a list of all EIGRP neighbors on a Cisco router. When you execute this command from the privileged EXEC mode, the router outputs a table that includes valuable details about each neighbor, such as: Neighbor Address: The IP address of the directly connected EIGRP neighbor. Interface: The interface on which the neighbor was learned. Hold Time: The time remaining before the neighbor is declared down if no hello packets are received. Uptime: How long the neighbor relationship has been maintained. Sequence Numbers: Information about the most recent update sequences that help maintain the routing table. Why This Command Is Important: Verifying neighbor relationships is a critical step in troubleshooting routing issues. If a router is not receiving updates from an expected neighbor, it could indicate a misconfiguration, physical connectivity issue, or a problem with EIGRP timers. The neighbor table also confirms that hello packets are being successfully exchanged, which is necessary for the stability of the routing protocol. How It Works: EIGRP uses periodic hello packets to establish and maintain neighbor relationships. These packets are sent on all interfaces enabled for EIGRP. When a router receives a hello packet, it checks for compatibility in parameters (such as autonomous system number and K-values) and, if matched, adds the sender to its neighbor table. The “show ip eigrp neighbors” command displays these relationships and helps ensure that the protocol is functioning as expected. Comparison with Other Options: Option B (show eigrp topology): This command displays the EIGRP topology table rather than neighbor relationships. The topology table includes routes that EIGRP has learned and is calculating the best path for, but it does not show details about directly connected neighbors. Option C (show ip route eigrp): This command filters the IP routing table for routes learned via EIGRP. It shows which routes are active in the routing table but does not provide information on neighbor adjacencies. Option D (show ip protocols): Although this command lists routing protocol information and some details about timers and configured networks, it does not provide the neighbor-level details that are necessary for EIGRP troubleshooting. Conclusion: The command “show ip eigrp neighbors” (Option A) is the standard tool used by network administrators to verify the status of EIGRP adjacencies. It provides real-time insight into the router’s neighbor relationships, which is critical for ensuring that EIGRP is operating correctly and that routing updates are being exchanged as expected. Mastery of this command is essential for troubleshooting and verifying the proper operation of EIGRP in a network.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.