Question

Exam 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.

Cisco Certified Design Expert CCDE v3.0 400-007 certification exam practice question and answer (Q&A) dump with detail explanation and reference available free, helpful to pass the Cisco Certified Design Expert CCDE v3.0 400-007 exam and earn Cisco Certified Design Expert CCDE v3.0 400-007 certification.

Correct Answer

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

Explanation 1

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

1. A feasible successor must be present.
2. The reported distance from a successor must be **lower** than the local feasible distance.

A feasible successor is a backup route that satisfies the feasibility condition. The feasibility condition states that the advertised distance (AD) of a neighbor must be less than the feasible distance (FD) of the current best path. If there is no feasible successor, EIGRP will query its neighbors for a new path to the destination network.

In summary, EIGRP can maintain an alternate loop-free path to a remote network if there is a feasible successor and the reported distance from a successor is lower than the local feasible distance.

Explanation 2

The correct answer is B and C.

EIGRP uses the concept of feasible successors to maintain alternate loop-free paths to a remote network. A feasible successor is a neighbor that has a lower Reported Distance (RD) than the local Feasible Distance (FD) to the destination network. The RD is the metric that a neighbor advertises for a network, and the FD is the best metric that the local router has for that network.

The feasibility condition states that for a neighbor to be a feasible successor, its RD must be lower than the FD of the current successor (the best path). This ensures that the alternate path does not have any loops, because it implies that the neighbor learned about the network from a different source than the current successor.

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 lower RD than the FD of the current successor.
• The RD from a successor must be lower than the local FD. This means that the current successor satisfies the feasibility condition and can be replaced by a feasible successor if it fails.

The other options are incorrect because:

• Option A is false because if the RD from a successor is higher than the local FD, it means that the successor does not satisfy the feasibility condition and cannot be replaced by a feasible successor.
• Option D is false because it confuses the RD and FD terms. The FD from a successor is always equal to or higher than its RD, because it includes the cost of reaching the successor itself.
• Option E is false because it contradicts the definition of a feasible successor and the feasibility condition.

Explanation 3

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 . 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.

Explanation 4

Explanation 5

The two conditions that must be met for EIGRP to maintain an alternate loop-free path to a remote network are:

1. A feasible successor must be present.
2. The Reported Distance from a successor is higher than the local Feasible Distance.

Explanation 6

Explanation 7

The correct answer is B and C.

Here is the explanation: EIGRP uses the DUAL algorithm to maintain a loop-free alternative path to a remote network. This path is called a feasible successor or a loop-free alternate (LFA). A feasible successor must meet the feasibility condition, which means that its reported distance (the distance advertised by the next hop) must be lower than the local feasible distance (the best metric to reach the destination). This ensures that the feasible successor is closer to the destination than the current router and does not depend on the current router for reaching the destination.

Therefore, to maintain an alternate loop-free path, EIGRP must have a feasible successor present (B) and its reported distance must be lower than the local feasible distance (C). Option A is incorrect because it implies that the successor is farther away from the destination than the current router, which violates the feasibility condition. Option D is incorrect because it confuses the reported distance and the feasible distance. Option E is incorrect because the feasibility condition must be met for an alternate loop-free path to exist.

Explanation 8

Explanation 9

The correct answers to the question are B. A feasible successor must be present, and C. The Reported Distance from a successor is lower than the local Feasible Distance.

To maintain an alternate loop-free path to a remote network in EIGRP, the following two conditions must be met:

B. A feasible successor must be present: A feasible successor is a backup route to a destination network that satisfies the feasibility condition. In EIGRP, for a route to be considered a feasible successor, its advertised distance (Reported Distance) must be less than the local router’s feasible distance. Feasible successors provide loop-free backup paths and allow for fast convergence in case the primary path (successor) fails.

C. The Reported Distance from a successor is lower than the local Feasible Distance: The reported distance is the metric advertised by a neighbor for a particular route. In EIGRP, for a route to be considered a feasible successor, the reported distance from the successor (the neighbor that provided the best path) must be lower than the local router’s feasible distance. This condition ensures that the feasible successor path is loop-free and can be used as a backup route.

These two conditions work together to establish and maintain alternate loop-free paths in EIGRP. By having feasible successors that meet the feasibility condition, the router can quickly switch to the backup path without causing routing loops or disruptions in the network.

The other options listed in the question are incorrect:

A. The Reported Distance from a successor is higher than the local Feasible Distance: If the reported distance from a successor is higher than the local router’s feasible distance, the route is not considered a feasible successor. This condition does not fulfill the requirements for maintaining an alternate loop-free path.

D. The Feasible Distance from a successor is lower than the local Reported Distance: This condition is the opposite of the requirement in EIGRP. The feasible distance from a successor should be higher than the reported distance to maintain a loop-free path.

E. The feasibility condition does not need to be met: This is incorrect. The feasibility condition must be met for a route to be considered a feasible successor in EIGRP. Feasible successors play a crucial role in providing loop-free backup paths and improving convergence time.

In summary, to maintain an alternate loop-free path to a remote network in EIGRP, a feasible successor must be present, and the reported distance from the successor must be lower than the local feasible distance. These conditions ensure that the backup path is valid, loop-free, and can be used for fast convergence in case of primary path failure.

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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