The answer is that mutual redistribution between OSPF and EIGRP without route tagging or filtering creates a routing loop. When a route learned from OSPF is redistributed into EIGRP, and then that same route is redistributed back into OSPF with a different metric, the router may prefer the redistributed route over the original, causing a feedback loop. This loop triggers continuous route updates, leading to CPU spikes and route flapping as the router oscillates between paths. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of redistribution dangers and the necessity of administrative distance manipulation or route tagging to prevent loops. A common trap is assuming that different protocol metrics alone prevent re-redistribution, but without explicit filtering, the router treats the redistributed route as a new, potentially better path. Remember the mnemonic: "Tag or filter, or your routes will flitter."
300-410 NAT and PAT Practice Question
This 300-410 practice question tests your understanding of nat and pat. 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. After a few minutes, the router's CPU spikes and routes start flapping. 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 redistribution is creating a routing loop because there is no route tagging or filtering to prevent re-redistribution.
Mutual redistribution without route tagging can cause a routing loop where a route redistributed from OSPF into EIGRP is then redistributed back into OSPF with a different metric, causing the router to prefer the redistributed route and create a loop. This leads to route flapping and CPU spikes as the router continuously updates.
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 redistribution is creating a routing loop because there is no route tagging or filtering to prevent re-redistribution.
Why this is correct
Correct. Without tagging, routes can loop between protocols.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The seed metric is not configured, so the routes are not redistributed.
Why it's wrong here
Missing seed metric would prevent redistribution, not cause flapping.
✗
The administrative distance is set too low, causing the router to prefer the wrong route.
Why it's wrong here
AD mismatch could cause issues but not necessarily loops.
✗
The OSPF process ID is the same on both routers.
Why it's wrong here
Process ID is local; same ID does not cause 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.
NAT and PAT — This question tests NAT and PAT — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistribution is creating a routing loop because there is no route tagging or filtering to prevent re-redistribution. — Mutual redistribution without route tagging can cause a routing loop where a route redistributed from OSPF into EIGRP is then redistributed back into OSPF with a different metric, causing the router to prefer the redistributed route and create a loop. This leads to route flapping and CPU spikes as the router continuously updates.
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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