The answer is to set appropriate administrative distance values for redistributed routes, as this is the most critical design consideration to prevent routing loops between OSPF and EIGRP. When redistributing between these protocols, a router can learn the same prefix from both OSPF (AD 110) and EIGRP (AD 90 for internal, 170 for external), and without adjusting the administrative distance, the router may prefer a redistributed route over a directly learned native route, creating a feedback loop where routes are re-injected into the original protocol. On the ENCOR 350-401 exam, this concept tests your understanding of how route preference and redistribution interact—a common trap is assuming that simply enabling route filtering or using route tags alone is sufficient, when in fact the default AD values can cause a router to choose a less trustworthy path. A reliable memory tip is to remember the phrase "redistributed routes must be less trusted than native routes," so always raise the AD of redistributed routes above the native protocol’s AD to break the loop.
CCNP Enterprise Network Design Practice Question
This 350-401 practice question tests your understanding of enterprise network design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 enterprise network uses OSPF in the core and EIGRP in the campus distribution layer. The engineer needs to redistribute routes between the two protocols. Which design consideration is most important to prevent routing loops?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set appropriate administrative distance values for redistributed routes.
Setting appropriate administrative distance values for redistributed routes is crucial to prevent routing loops when redistributing between OSPF and EIGRP. By default, EIGRP has an administrative distance of 170 for external routes and 90 for internal routes, while OSPF uses 110. If redistributed routes are not assigned a higher administrative distance, a router might prefer a redistributed route over a directly learned route, creating a feedback loop where routes are re-injected into the original protocol. Adjusting the administrative distance ensures that redistributed routes are less preferred than native routes, breaking the loop.
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.
✓
Set appropriate administrative distance values for redistributed routes.
Why this is correct
Correct because AD controls route preference; if not set correctly, a redistributed route could be preferred over the original, causing loops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use route maps to filter all redistributed routes.
Why it's wrong here
Incorrect while route maps can filter routes, they do not directly prevent loops; AD and metric settings are more critical.
✗
Enable OSPF on all EIGRP interfaces.
Why it's wrong here
Incorrect because running both protocols on the same interface does not prevent loops and can cause confusion.
✗
Use a single routing protocol throughout the network.
Why it's wrong here
Incorrect while this would avoid redistribution, the scenario requires using both protocols, so it is not a valid solution.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that route filtering (option B) is the primary loop-prevention mechanism, but the real trap is that administrative distance must be adjusted to prevent the redistribution feedback loop, especially when multiple routers perform mutual redistribution.
Trap categories for this question
Similar concept trap
Incorrect because running both protocols on the same interface does not prevent loops and can cause confusion.
Scenario analysis trap
Incorrect while this would avoid redistribution, the scenario requires using both protocols, so it is not a valid solution.
Detailed technical explanation
How to think about this question
Under the hood, OSPF and EIGRP use different metrics (cost vs. composite metric), so redistribution requires a seed metric. Without adjusting administrative distance, a router could learn an OSPF external route via EIGRP with a lower AD (e.g., 170 vs. 110) and re-advertise it back into OSPF, creating a routing loop. Cisco recommends using the `distance` command under the routing process or the `redistribute` command with a higher distance value to prevent this. In real-world scenarios, this is critical when redistributing between OSPF and EIGRP in a hub-and-spoke topology where multiple redistribution points exist.
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.
Enterprise Network Design — This question tests Enterprise Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Set appropriate administrative distance values for redistributed routes. — Setting appropriate administrative distance values for redistributed routes is crucial to prevent routing loops when redistributing between OSPF and EIGRP. By default, EIGRP has an administrative distance of 170 for external routes and 90 for internal routes, while OSPF uses 110. If redistributed routes are not assigned a higher administrative distance, a router might prefer a redistributed route over a directly learned route, creating a feedback loop where routes are re-injected into the original protocol. Adjusting the administrative distance ensures that redistributed routes are less preferred than native routes, breaking the loop.
What should I do if I get this 350-401 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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