Question 1,262 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

Routing Loop from Mutual Redistribution

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.

A
The redistribution is creating a routing loop because there is no route tagging or filtering to prevent re-redistribution.
Correct. Without tagging, routes can loop between protocols.
B
The seed metric is not configured, so the routes are not redistributed.
Why wrong: Missing seed metric would prevent redistribution, not cause flapping.
C
The administrative distance is set too low, causing the router to prefer the wrong route.
Why wrong: AD mismatch could cause issues but not necessarily loops.
D
The OSPF process ID is the same on both routers.
Why wrong: Process ID is local; same ID does not cause loops.

Full breakdown with real-world context →

Quick Answer

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."

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

✓

The redistribution is creating a routing loop because there is no route tagging or filtering to prevent re-redistribution.

Option A is correct because mutual redistribution without route tagging or filtering causes the redistributed routes to be re-injected back into the original routing protocol, creating a routing loop. This loop leads to continuous route updates, CPU spikes, and route flapping as the router repeatedly processes and advertises the same prefixes.

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 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.
Related concept
Read the scenario before looking for a memorised 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: answer the scenario, not the keyword

Cisco often tests the concept that mutual redistribution inherently causes loops unless explicit filtering or tagging is applied, and candidates mistakenly focus on missing metrics or administrative distance instead of the re-redirection loop.

Detailed technical explanation

How to think about this question

Mutual redistribution between OSPF and EIGRP requires route tagging (e.g., using a route-map with a tag) or filtering (e.g., distribute-list) to prevent routes learned from one protocol from being advertised back into it. Without these safeguards, a route redistributed from EIGRP into OSPF can be learned by the same router via OSPF and then redistributed back into EIGRP, creating a two-way loop. This is a classic example of a redistribution feedback loop, often mitigated by setting administrative distance or using route tags with deny statements.

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.

Visual reference

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

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?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

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. — Option A is correct because mutual redistribution without route tagging or filtering causes the redistributed routes to be re-injected back into the original routing protocol, creating a routing loop. This loop leads to continuous route updates, CPU spikes, and route flapping as the router repeatedly processes and advertises the same prefixes.

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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