- A
The seed metric for EIGRP was not configured, causing the route to be redistributed with an infinite metric.
Why wrong: Missing seed metric prevents redistribution, not causes loops.
- B
The administrative distance of the redistributed routes is lower than the original, causing them to be preferred.
Why wrong: AD does not change during redistribution unless explicitly modified; the loop is due to re-redistribution.
- C
Routes redistributed from OSPF into EIGRP are then redistributed back into OSPF, creating a feedback loop.
Without filtering or tagging, redistributed routes can be re-injected into the original protocol, causing instability.
- D
The OSPF process ID must match on all routers; otherwise, redistribution fails.
Why wrong: Process ID is local; redistribution does not require matching IDs.
Quick Answer
The answer is that routes redistributed from OSPF into EIGRP are then redistributed back into OSPF, creating a feedback loop. This occurs because when mutual redistribution is configured without route tagging or filtering, a route learned from OSPF is injected into EIGRP, and since EIGRP has no way to distinguish it from a native EIGRP route, it can be redistributed back into OSPF, causing the same route to be re-advertised in a cycle. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of redistribution loop prevention, often appearing as a classic trap where engineers forget to use route maps or administrative distance adjustments. A common memory tip is to think of it as a “ping-pong” effect—the route bounces between protocols endlessly. Remember the mnemonic “TAG it to stop the loop”: Tag, Administrative distance, or Filtering are the three tools to break the cycle.
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures mutual redistribution between OSPF and EIGRP on a router. After a few minutes, the router's CPU spikes and routing loops occur. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
Routes redistributed from OSPF into EIGRP are then redistributed back into OSPF, creating a feedback loop.
Mutual redistribution without route tagging or filtering can cause a routing loop. A route redistributed from OSPF into EIGRP can be redistributed back into OSPF, creating a feedback loop. This is a classic redistribution loop.
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.
- ✗
The seed metric for EIGRP was not configured, causing the route to be redistributed with an infinite metric.
Why it's wrong here
Missing seed metric prevents redistribution, not causes loops.
- ✗
The administrative distance of the redistributed routes is lower than the original, causing them to be preferred.
Why it's wrong here
AD does not change during redistribution unless explicitly modified; the loop is due to re-redistribution.
- ✓
Routes redistributed from OSPF into EIGRP are then redistributed back into OSPF, creating a feedback loop.
Why this is correct
Without filtering or tagging, redistributed routes can be re-injected into the original protocol, causing instability.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The OSPF process ID must match on all routers; otherwise, redistribution fails.
Why it's wrong here
Process ID is local; redistribution does not require matching IDs.
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 300-410 OSPF questions on adjacency and route selection.
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Administrative Distance — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
Administrative Distance — This question tests Administrative Distance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Routes redistributed from OSPF into EIGRP are then redistributed back into OSPF, creating a feedback loop. — Mutual redistribution without route tagging or filtering can cause a routing loop. A route redistributed from OSPF into EIGRP can be redistributed back into OSPF, creating a feedback loop. This is a classic redistribution loop.
What should I do if I get this 300-410 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 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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