- A
The seed metric for EIGRP redistribution is set to a low value, causing EIGRP routes to be preferred over OSPF routes.
Why wrong: Seed metric affects path selection but does not directly cause loops; loops occur due to re-redistribution.
- B
Routes redistributed from OSPF into EIGRP are re-distributed back into OSPF because there is no route tagging to identify them as OSPF-originated.
Without route tagging, the router sees the redistributed routes as EIGRP routes and redistributes them back into OSPF, causing the same prefix to be advertised back to the original OSPF domain, leading to loops.
- C
The IPsec tunnel is using transport mode, which causes routing protocol packets to be dropped.
Why wrong: Transport mode does not affect routing protocol behavior; loops are a redistribution issue.
- D
The OSPF process has a higher administrative distance than EIGRP, causing route flapping.
Why wrong: AD differences affect path selection but do not cause loops; loops are caused by re-redistribution.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 that is part of an IPsec site-to-site VPN. After the configuration, routing loops occur intermittently. The engineer has not used any route tagging. What is the most likely cause of the routing loops?
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 re-distributed back into OSPF because there is no route tagging to identify them as OSPF-originated.
Without route tagging, routes redistributed from OSPF into EIGRP are not marked as OSPF-originated. When EIGRP redistributes these routes back into OSPF, OSPF accepts them as external routes, creating a mutual redistribution loop. This occurs because OSPF has no mechanism to distinguish between its own routes and those learned from EIGRP without explicit tagging (e.g., using a route-map with a tag).
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 seed metric for EIGRP redistribution is set to a low value, causing EIGRP routes to be preferred over OSPF routes.
Why it's wrong here
Seed metric affects path selection but does not directly cause loops; loops occur due to re-redistribution.
- ✓
Routes redistributed from OSPF into EIGRP are re-distributed back into OSPF because there is no route tagging to identify them as OSPF-originated.
Why this is correct
Without route tagging, the router sees the redistributed routes as EIGRP routes and redistributes them back into OSPF, causing the same prefix to be advertised back to the original OSPF domain, leading to 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 IPsec tunnel is using transport mode, which causes routing protocol packets to be dropped.
Why it's wrong here
Transport mode does not affect routing protocol behavior; loops are a redistribution issue.
- ✗
The OSPF process has a higher administrative distance than EIGRP, causing route flapping.
Why it's wrong here
AD differences affect path selection but do not cause loops; loops are caused by re-redistribution.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that mutual redistribution without route tagging or filtering is the primary cause of routing loops, and candidates mistakenly focus on metric or administrative distance differences instead of the re-injection mechanism.
Detailed technical explanation
How to think about this question
In mutual redistribution, without route tagging (e.g., using the 'tag' option in a route-map or the 'default-metric' command), OSPF routes redistributed into EIGRP lose their origin identity. When EIGRP redistributes its routes (including those learned from OSPF) back into OSPF, OSPF treats them as new external routes, potentially with a better metric, causing them to be preferred over the original OSPF routes. This creates a feedback loop where routes are continuously re-injected, leading to intermittent routing loops. A common mitigation is to set a route tag on redistributed routes and filter them using a distribute-list or route-map to prevent re-injection.
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?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Routes redistributed from OSPF into EIGRP are re-distributed back into OSPF because there is no route tagging to identify them as OSPF-originated. — Without route tagging, routes redistributed from OSPF into EIGRP are not marked as OSPF-originated. When EIGRP redistributes these routes back into OSPF, OSPF accepts them as external routes, creating a mutual redistribution loop. This occurs because OSPF has no mechanism to distinguish between its own routes and those learned from EIGRP without explicit tagging (e.g., using a route-map with a tag).
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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