CCDE 400-007: Which conditions must be met for EIGRP to maintain alternate loop-free path to remote network?

Question

Which two conditions must be met for EIGRP to maintain an alternate loop-free path to a remote network? (Choose two.)

A. The Reported Distance from a successor is higher than the local Feasible Distance.
B. A feasible successor must be present.
C. The Reported Distance from a successor is lower than the local Feasible Distance.
D. The Feasible Distance from a successor is lower than the local Reported Distance.
E. The feasibility condition does not need to be met.

Answer

B. A feasible successor must be present.
C. The Reported Distance from a successor is lower than the local Feasible Distance.

Explanation

The correct answer is B and C. Here is the explanation:

EIGRP uses the Diffusing Update Algorithm (DUAL) to select loop-free paths to a destination network. A loop-free path is one that does not contain a router that is closer to the destination than the current router. EIGRP maintains a topology table that contains all the paths learned from its neighbors, along with their metrics and feasibility conditions.

The metric of a path is the sum of the delays and bandwidths of all the links along the path. The metric of a path reported by a neighbor is called the Reported Distance (RD). The metric of a path as seen by the local router is called the Feasible Distance (FD).

A feasible successor is a neighbor that has a loop-free path to the destination and satisfies the feasibility condition, which is: RD < FD. This means that the neighbor’s distance to the destination is less than the local router’s distance to the destination.

A successor is the neighbor that provides the best loop-free path to the destination, which has the lowest FD among all feasible successors. EIGRP installs the successor’s route in the routing table and uses it for packet forwarding.

If the successor fails or becomes unreachable, EIGRP can quickly switch to a feasible successor without recalculating the route, if one exists. This provides fast convergence and reduces routing traffic.

Therefore, for EIGRP to maintain an alternate loop-free path to a remote network, two conditions must be met:

• A feasible successor must be present. This means that there is at least one neighbor that has a loop-free path to the destination and satisfies the feasibility condition.
• The Reported Distance from a successor is lower than the local Feasible Distance. This means that the successor’s distance to the destination is less than the local router’s distance to the destination.

The other options are incorrect because:

• A. The Reported Distance from a successor is higher than the local Feasible Distance. This violates the feasibility condition and implies that there is a routing loop.
• D. The Feasible Distance from a successor is lower than the local Reported Distance. This is not possible because FD = RD + link cost, so FD must be greater than or equal to RD.
• E. The feasibility condition does not need to be met. This contradicts the definition of a feasible successor and may result in routing loops.

Reference

• EIGRP Loop-Free Alternate Fast Reroute  [Cisco IOS 15.2S] – Cisco Systems
• What’s EIGRP? What’s OSPF? EIGRP vs OSPF Differences | FS Community
• EIGRP Loop-Free Alternate (LFA) Fast Reroute (FRR) (networklessons.com)
• Understand and Use the Enhanced Interior Gateway Routing Protocol – Cisco
• Introduction to EIGRP – Cisco

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