- A
Routes redistributed from OSPF into EIGRP have a higher administrative distance than the original OSPF routes, causing them to be preferred.
Why wrong: Administrative distance is compared when the same prefix is learned from multiple sources, but the loop occurs because the route is re-injected.
- B
Redistributed routes retain their original metric, which can cause them to be preferred over the original route.
Why wrong: Metric is compared within the same protocol; the loop is due to re-redistribution, not metric preference.
- C
Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle.
Route tagging (e.g., using a route-map) is essential to prevent redistributed routes from being re-redistributed. Without it, each protocol can re-inject the other's routes, causing loops.
- D
The seed metric for redistribution is not set, causing the redistributed routes to be rejected.
Why wrong: A missing seed metric would prevent redistribution, not cause loops. The loops occur because redistribution succeeds but without filtering.
Quick Answer
The correct answer is that mutual redistribution without route tagging creates a feedback loop where OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, causing a cycle. This happens because each protocol treats the redistributed routes as external and re-advertises them into the other protocol, leading to continuous re-injection and routing loops. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of redistribution mechanics and the critical need for loop prevention; a common trap is assuming administrative distance alone will break the cycle, but without tagging, the routes are simply re-learned and re-advertised indefinitely. A useful memory tip is "no tag, no break"—if you forget to tag routes during mutual redistribution, you create a routing loop that loops back on itself like a snake eating its own tail.
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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. After a few minutes, routing loops occur. The engineer did not use route tagging. 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
Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle.
Option C is correct because mutual redistribution without route tagging creates a feedback loop: OSPF redistributes EIGRP-learned routes back into OSPF, and EIGRP redistributes OSPF-learned routes back into EIGRP. Each protocol re-advertises the other's routes, causing them to be learned and re-injected repeatedly, which leads to routing loops. Route tagging (e.g., using a route-map to set a tag) is the standard method to prevent such cycles by filtering redistributed routes that already originated from the other protocol.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Routes redistributed from OSPF into EIGRP have a higher administrative distance than the original OSPF routes, causing them to be preferred.
Why it's wrong here
Administrative distance is compared when the same prefix is learned from multiple sources, but the loop occurs because the route is re-injected.
- ✗
Redistributed routes retain their original metric, which can cause them to be preferred over the original route.
Why it's wrong here
Metric is compared within the same protocol; the loop is due to re-redistribution, not metric preference.
- ✓
Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle.
Why this is correct
Route tagging (e.g., using a route-map) is essential to prevent redistributed routes from being re-redistributed. Without it, each protocol can re-inject the other's routes, causing loops.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The seed metric for redistribution is not set, causing the redistributed routes to be rejected.
Why it's wrong here
A missing seed metric would prevent redistribution, not cause loops. The loops occur because redistribution succeeds but without filtering.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that routing loops in mutual redistribution are caused by administrative distance or metric issues, when in fact the core problem is the lack of route tagging to prevent re-redistribution of routes back into their original protocol.
Detailed technical explanation
How to think about this question
Under the hood, OSPF redistributes EIGRP routes as Type 5 external LSAs (or Type 7 in NSSA), and EIGRP redistributes OSPF routes as external EIGRP routes. Without a route tag, there is no mechanism to distinguish a route that was originally from OSPF versus one that was learned via EIGRP and then re-injected. In real-world scenarios, engineers often use a route-map with a 'tag' option (e.g., set tag 100) and then match that tag to deny redistribution, effectively breaking the loop. Cisco's documentation recommends always using route tagging when performing mutual redistribution between any two IGPs.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle. — Option C is correct because mutual redistribution without route tagging creates a feedback loop: OSPF redistributes EIGRP-learned routes back into OSPF, and EIGRP redistributes OSPF-learned routes back into EIGRP. Each protocol re-advertises the other's routes, causing them to be learned and re-injected repeatedly, which leads to routing loops. Route tagging (e.g., using a route-map to set a tag) is the standard method to prevent such cycles by filtering redistributed routes that already originated from the other protocol.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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