- A
A feasible successor must have a reported distance less than the feasible distance.
Correct because the feasibility condition requires the reported distance (neighbor's metric) to be strictly less than the feasible distance (the current best metric).
- B
A feasible successor is immediately used when the successor fails, without any query process.
Correct because EIGRP can instantly switch to a feasible successor because it is guaranteed to be loop-free.
- C
The feasible distance is the metric of the feasible successor route.
Why wrong: Incorrect because the feasible distance is the metric of the successor (best) route, not the feasible successor.
- D
EIGRP will always have at least one feasible successor for every route.
Why wrong: Incorrect because feasible successors are not guaranteed; they depend on the network topology and the feasibility condition.
- E
The feasible successor is stored in the routing table as a backup route.
Why wrong: Incorrect because feasible successors are stored in the topology table, not the routing table. Only the successor is placed in the routing table.
Quick Answer
The answer is that a feasible successor is immediately used when the successor fails, without any query process. This is correct because the feasible successor meets the EIGRP feasible successor condition, which requires that the reported distance from the neighbor is strictly less than the feasible distance (the lowest metric to the destination). This condition ensures the backup route is loop-free, allowing the router to instantly promote the feasible successor from the topology table to the routing table upon successor failure, avoiding the delay of querying neighbors. On the ENCOR 350-401 exam, this concept tests your understanding of EIGRP’s Diffusing Update Algorithm (DUAL) and the distinction between the successor, feasible successor, and the query process. A common trap is confusing the reported distance with the feasible distance or assuming all backup routes are feasible successors. Memory tip: think “RD < FD” for a backup that is ready—no queries, just immediate use.
350-401 EIGRP Practice Question
This 350-401 practice question tests your understanding of eigrp. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which two statements about EIGRP feasible successors are true? (Choose two.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A feasible successor must have a reported distance less than the feasible distance.
A feasible successor is a backup route that meets the feasibility condition (reported distance < feasible distance). It is stored in the topology table and can be used immediately if the successor fails, without querying neighbors. The feasible distance is the lowest metric to a destination; the successor is the route with that metric. The reported distance is the neighbor's metric to the destination.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A feasible successor must have a reported distance less than the feasible distance.
- ✓
A feasible successor is immediately used when the successor fails, without any query process.
- ✗
The feasible distance is the metric of the feasible successor route.
- ✗
EIGRP will always have at least one feasible successor for every route.
Why it's wrong here
Incorrect because feasible successors are not guaranteed; they depend on the network topology and the feasibility condition.
- ✗
The feasible successor is stored in the routing table as a backup route.
Why it's wrong here
Incorrect because feasible successors are stored in the topology table, not the routing table. Only the successor is placed in the routing table.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
EIGRP — This question tests EIGRP — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: A feasible successor must have a reported distance less than the feasible distance. — A feasible successor is a backup route that meets the feasibility condition (reported distance < feasible distance). It is stored in the topology table and can be used immediately if the successor fails, without querying neighbors. The feasible distance is the lowest metric to a destination; the successor is the route with that metric. The reported distance is the neighbor's metric to the destination.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Same concept, more angles
1 more ways this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which two statements about EIGRP feasible successors are true? (Choose two.)
medium- ✓ A.A feasible successor must have a reported distance less than the feasible distance of the current successor.
- B.A feasible successor is installed in the routing table as a backup route.
- ✓ C.If a successor fails, EIGPR immediately uses a feasible successor without transitioning to active state.
- D.The feasible successor must have the same metric as the successor.
- E.EIGRP uses the Diffusing Update Algorithm (DUAL) to determine feasible successors.
Why A: A feasible successor is a backup route that meets the feasibility condition (reported distance < feasible distance). It is stored in the topology table, not the routing table, and is used immediately if the successor fails.
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Last reviewed: Jun 18, 2026
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