In EIGRP, a route is considered feasible only if the reported distance from the neighbor is less than which of the following?
Correct Answer: B
Detailed Explanation: Enhanced Interior Gateway Routing Protocol (EIGRP) uses a composite metric to determine the best routes to destination networks. One key concept in EIGRP is the feasibility condition, which is used to ensure loop-free routing and guarantee that a route is a valid backup path. The feasibility condition requires that for a route to be considered a valid successor (or a feasible successor), the reported distance (RD) from the neighboring router must be less than the feasible distance (FD) of the local router for that route. Here’s what these terms mean: Feasible Distance (FD): The FD is the lowest composite metric from the local router to the destination network, including the metric from the local router’s perspective. It represents the best-known cost to reach that destination. Reported Distance (RD): The RD is the metric value that a neighboring router advertises for a particular route. It reflects the neighbor’s perspective of the cost from itself to the destination. For a route to be loop-free, EIGRP requires that the neighbor’s RD be less than the local router’s FD. This condition ensures that the neighbor’s path to the destination does not include a loop back to the local router. In simpler terms, if a neighbor reports a metric that is lower than the best metric the local router currently has, that route can be safely used as a backup (feasible successor). Why the Feasibility Condition Matters: The feasibility condition is critical in EIGRP because it allows the protocol to maintain a set of backup routes that are guaranteed to be loop-free. This contributes to fast convergence since the router can immediately switch to a feasible successor if the primary (successor) route fails, without needing to recalculate the entire topology. Other Options Explained: Administrative Distance (Option A): Administrative distance is a value used to determine the trustworthiness of a routing protocol, not a component of the EIGRP metric calculation or feasibility condition. Maximum Hop Count (Option C): EIGRP does not use a simple hop count; instead, it relies on a composite metric based on factors like bandwidth and delay. Variance (Option D): Variance is a parameter used in EIGRP to control load balancing over unequal-cost paths, but it is not part of the feasibility condition. Practical Implications: Network engineers rely on the feasibility condition to troubleshoot EIGRP routing issues. If a feasible successor is not available, the network may experience slower convergence when a primary route fails. Tools such as the “show ip eigrp topology” command display the FD and RD values, allowing administrators to verify that the feasibility condition is being met for backup routes. Conclusion: In EIGRP, a route is considered feasible if the reported distance from the neighbor is less than the feasible distance (Option B). This condition is fundamental for ensuring loop-free and reliable routing, making it a key concept in understanding and troubleshooting EIGRP operations.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.