The answer is that the redistribute command without route-map or tag allows routes to be re-advertised back into the source protocol, creating a loop. In mutual redistribution, when route tagging is absent, a route learned from OSPF and redistributed into EIGRP can be re-injected back into OSPF by the same or another router, causing a feedback loop where the same prefix circulates endlessly between the two protocols. This is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding that administrative distance differences (OSPF 110 vs. EIGRP 90/170) may influence path selection but are not the root cause of the loop—the lack of tagging and filtering is. The exam often presents a scenario where an engineer forgets to apply a route-map with a tag, leading to this exact routing loop in mutual redistribution. Memory tip: "No tag, no stop—routes loop back to the top."
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. 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 experienced network engineer configures mutual redistribution between OSPF and EIGRP on a router. Both protocols have routes to the same prefix, but after redistribution, a routing loop occurs. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The redistribute command without route-map or tag allows routes to be re-advertised back into the source protocol, creating a loop.
Without route tagging, redistributed routes are re-injected back into the original protocol, causing a feedback loop. Administrative distance comparison (OSPF 110 vs EIGRP 90/170) can cause suboptimal path selection, but the loop is primarily due to lack of tagging and filtering.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 was not configured, causing routes to be rejected.
Why it's wrong here
Missing seed metric prevents redistribution, not loops.
✓
The redistribute command without route-map or tag allows routes to be re-advertised back into the source protocol, creating a loop.
Why this is correct
Mutual redistribution without tagging and filtering causes a feedback loop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
OSPF has a lower administrative distance than EIGRP, so OSPF routes are always preferred.
Why it's wrong here
AD affects path selection but does not directly cause redistribution loops.
✗
EIGRP stub configuration on the redistributing router prevents routes from being advertised.
Why it's wrong here
Stub restricts query propagation, not redistribution loops.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistribute command without route-map or tag allows routes to be re-advertised back into the source protocol, creating a loop. — Without route tagging, redistributed routes are re-injected back into the original protocol, causing a feedback loop. Administrative distance comparison (OSPF 110 vs EIGRP 90/170) can cause suboptimal path selection, but the loop is primarily due to lack of tagging and filtering.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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