- A
The route-map OSPF-to-EIGRP on R1 should include a match clause to filter out routes learned from R2 via OSPF, preventing feedback.
Adding a match route-map that tags routes from R2 and denies them, or using a tag-based filter, stops the redistribution loop.
- B
The redistribute command on R2 should have a metric-type 1 to avoid suboptimal routing.
Why wrong: Metric type does not prevent loops; it only affects OSPF path selection.
- C
The administrative distance of OSPF should be changed to 85 on R1 to prefer OSPF over EIGRP.
Why wrong: Changing AD may hide the symptom but does not fix the loop; the loop will still exist and may cause other issues.
- D
The EIGRP metric on R1 should be set to a higher value to make the redistributed route less preferred.
Why wrong: Higher metric does not prevent redistribution of the route back into OSPF; the loop persists.
Quick Answer
The answer is that the route-map OSPF-to-EIGRP on R1 lacks a match clause to filter out the OSPF route 172.16.1.0/24, which was originally redistributed from EIGRP by R2. This omission causes a routing feedback loop: R1 learns the prefix via OSPF, redistributes it back into EIGRP with a better administrative distance (90 vs. 110), and R2 then prefers the EIGRP path, breaking reachability and creating loops. On the Cisco CCNP ENARSI 300-410 exam, mutual redistribution between OSPF and EIGRP frequently tests your ability to prevent route feedback using route-map filtering, often with tags or prefix-lists to distinguish redistributed routes. A common trap is assuming that simply applying a route-map is enough—without explicit match conditions, the redistribution is unfiltered. Memory tip: "Tag it or bag it"—always tag redistributed routes or use a prefix-list to block the feedback path, ensuring one protocol never re-injects routes it originally learned from the other.
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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.
A network engineer is troubleshooting a redistribution issue between OSPF and EIGRP. Router R1 redistributes OSPF into EIGRP, and Router R2 redistributes EIGRP into OSPF. After configuration, some routes are missing, and routing loops occur. R1 has: router eigrp 100 redistribute ospf 1 metric 10000 100 255 1 1500 route-map OSPF-to-EIGRP. R2 has: router ospf 1 redistribute eigrp 100 subnets route-map EIGRP-to-OSPF. Show ip route on R1 shows an OSPF route 172.16.1.0/24 learned via R2, but also an EIGRP route for the same prefix with a better administrative distance. What is the root cause?
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
The route-map OSPF-to-EIGRP on R1 should include a match clause to filter out routes learned from R2 via OSPF, preventing feedback.
The correct answer is A because the route-map OSPF-to-EIGRP on R1 is not filtering out the OSPF route 172.16.1.0/24 that was originally redistributed from EIGRP into OSPF by R2. This creates a routing feedback loop: R1 learns the prefix via OSPF (from R2) and then redistributes it back into EIGRP, causing R2 to see an EIGRP route with a better administrative distance (90 vs. 110) and prefer it, leading to missing routes and loops. The route-map should include a match clause to deny routes that were originally EIGRP (e.g., via a tag or prefix-list) to prevent mutual redistribution.
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.
- ✓
The route-map OSPF-to-EIGRP on R1 should include a match clause to filter out routes learned from R2 via OSPF, preventing feedback.
Why this is correct
Adding a match route-map that tags routes from R2 and denies them, or using a tag-based filter, stops the redistribution loop.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The redistribute command on R2 should have a metric-type 1 to avoid suboptimal routing.
- ✗
The administrative distance of OSPF should be changed to 85 on R1 to prefer OSPF over EIGRP.
Why it's wrong here
Changing AD may hide the symptom but does not fix the loop; the loop will still exist and may cause other issues.
- ✗
The EIGRP metric on R1 should be set to a higher value to make the redistributed route less preferred.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept of mutual redistribution and route feedback loops, where candidates mistakenly focus on metric or administrative distance adjustments instead of recognizing that a route-map filter is required to break the redistribution cycle.
Detailed technical explanation
How to think about this question
Mutual redistribution between OSPF and EIGRP requires careful route filtering to prevent routing loops, typically using route tags or prefix-lists. In this scenario, the OSPF route 172.16.1.0/24 on R1 has an administrative distance of 110, while the same prefix learned via EIGRP has an AD of 90, so EIGRP is preferred; this causes R1 to install the EIGRP route and potentially forward traffic incorrectly, while R2 may also receive the redistributed route back, creating a loop. A common best practice is to tag routes at the point of redistribution and then filter based on that tag in the opposite direction to avoid feedback.
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
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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: The route-map OSPF-to-EIGRP on R1 should include a match clause to filter out routes learned from R2 via OSPF, preventing feedback. — The correct answer is A because the route-map OSPF-to-EIGRP on R1 is not filtering out the OSPF route 172.16.1.0/24 that was originally redistributed from EIGRP into OSPF by R2. This creates a routing feedback loop: R1 learns the prefix via OSPF (from R2) and then redistributes it back into EIGRP, causing R2 to see an EIGRP route with a better administrative distance (90 vs. 110) and prefer it, leading to missing routes and loops. The route-map should include a match clause to deny routes that were originally EIGRP (e.g., via a tag or prefix-list) to prevent mutual redistribution.
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.
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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