The answer is to use route-maps to tag EIGRP routes and filter them on OSPF routers. This configuration prevents redistribution loops by assigning a specific tag, such as 'tag 100', to routes redistributed from EIGRP into OSPF, and then applying a distribute-list or prefix-list on the OSPF side to block any route carrying that tag from being re-injected back into EIGRP. Without this tagging and filtering mechanism, mutual redistribution between EIGRP and OSPF creates a two-way feedback loop where routes bounce between the two protocols, causing instability and potential black holes. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of route redistribution loop prevention, often appearing in migration questions where OSPF area 0 and area 1 interact with EIGRP AS 100. A common trap is assuming that administrative distance alone will break the loop, but route tagging is required because distance does not prevent re-advertisement. Remember the memory tip: “Tag it once, block it twice”—tag the redistributed routes at the source, then filter them on the return path to keep the loop broken.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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.
During a network migration from EIGRP to OSPF, you notice that some routes are being redistributed incorrectly, causing routing loops. The OSPF domain uses area 0 and area 1. The EIGRP domain uses AS 100. Which configuration change would best prevent loops during the migration?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use route-maps to tag EIGRP routes and filter them on OSPF routers.
Option B is correct because route-maps allow you to tag redistributed EIGRP routes with a specific tag value (e.g., 'tag 100') and then filter those tagged routes on OSPF routers using a distribute-list in or prefix-list combined with the route-map. This prevents the redistributed routes from being re-injected back into EIGRP, breaking the redistribution loop. Without such tagging and filtering, mutual redistribution between EIGRP and OSPF can cause routing loops due to the two-way redistribution of routes.
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.
✗
Implement OSPF stub areas to limit external routes.
Why it's wrong here
Stub areas limit external LSAs but do not prevent redistribution loops.
✓
Use route-maps to tag EIGRP routes and filter them on OSPF routers.
Why this is correct
Tags allow conditional redistribution filtering, preventing routes from being sent back to EIGRP.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use distribute-list in EIGRP to block OSPF routes.
Why it's wrong here
While this can prevent loops, route-maps with tags are more granular and a best practice.
✗
Set a high administrative distance on redistributed routes in OSPF.
Why it's wrong here
Changing AD might cause preference issues but does not prevent loops.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that simply adjusting administrative distance or using stub areas can prevent redistribution loops, when in fact only explicit tagging and filtering (or route-map-based control) can break the two-way redistribution cycle.
Detailed technical explanation
How to think about this question
Under the hood, mutual redistribution between EIGRP and OSPF creates a feedback loop where a route learned via OSPF is redistributed into EIGRP, then learned back by OSPF via EIGRP redistribution, causing a cycle. The standard solution is to use route tagging (e.g., 'set tag 100' on redistributed EIGRP routes) and then filter those tagged routes on OSPF routers using a route-map with a 'match tag' clause in a distribute-list or prefix-list. In real-world migrations, this technique is combined with administrative distance adjustments (e.g., setting distance 200 on redistributed routes) to provide additional safety, but tagging and filtering is the primary loop-prevention mechanism.
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.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use route-maps to tag EIGRP routes and filter them on OSPF routers. — Option B is correct because route-maps allow you to tag redistributed EIGRP routes with a specific tag value (e.g., 'tag 100') and then filter those tagged routes on OSPF routers using a distribute-list in or prefix-list combined with the route-map. This prevents the redistributed routes from being re-injected back into EIGRP, breaking the redistribution loop. Without such tagging and filtering, mutual redistribution between EIGRP and OSPF can cause routing loops due to the two-way redistribution of routes.
What should I do if I get this 350-501 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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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