Question 7 of 2,152

EIGRP Troubleshooting →hardMultiple ChoiceObjective-mapped

Understanding the EIGRP Null0 Route in the Topology Table

This 300-410 practice question tests your understanding of eigrp troubleshooting. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.

A network engineer runs the following command on Router R1:

R1# show ip eigrp topology 10.10.10.0/24

EIGRP-IPv4 Topology Entry for AS(100)/ID(192.168.1.1) for 10.10.10.0/24 State: Passive, Reply status: 0, Originating router: 192.168.1.1 Routing Descriptor Blocks:

0.0.0.0 (Null0) from 0.0.0.0, Send flag: 0x0

Composite metric: (2560000000/0), Route is Internal Vector metric: Minimum bandwidth: 100000 Kbit Total delay: 100 microseconds Reliability: 255/255 Load: 1/255 Minimum MTU: 1500 Hop count: 0

Based on this output, what is the problem?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.

A
The route is a local summary route pointing to Null0, which is normal for EIGRP summarization.
EIGRP automatically installs a null0 route for summary addresses to prevent loops.
B
The route has a metric of 2560000000, indicating a network failure.
Why wrong: A high metric for a null0 route is expected and does not indicate failure.
C
The hop count of 0 means the route is not reachable.
Why wrong: Hop count of 0 is normal for a directly connected or summary route.
D
The route is in Active state, indicating a problem.
Why wrong: The state is Passive, which is normal.

Full breakdown with real-world context →

Quick Answer

The answer is that there is no problem; the EIGRP Null0 summary route in the topology table is a normal and expected entry. This route appears because EIGRP automatically installs a local summary route pointing to Null0 whenever a manual summary is configured or when a directly connected network is advertised, using a composite metric of 2560000000/0 to prevent routing loops by discarding packets that match the summary but not a more specific route. On the Cisco CCNP ENARSI 300-410 exam, this output tests your ability to distinguish between a legitimate Null0 route and a problematic black-hole scenario, often appearing as a distractor where candidates mistakenly think the high metric indicates a failure. A common trap is assuming any route with a 2560000000 metric is broken, but remember: a Null0 route with a hop count of 0 and an originating router matching the local router ID is perfectly healthy. Memory tip: “Null0 is normal for summaries—high metric, zero hops, no problem.”

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 route is a local summary route pointing to Null0, which is normal for EIGRP summarization.

The output shows a Null0 summary route in EIGRP, which is normal when manual summarization is configured. The route is in Passive state with a composite metric of 2560000000/0, indicating it is a local summary route installed to prevent routing loops. This is expected behavior, not a problem.

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 route is a local summary route pointing to Null0, which is normal for EIGRP summarization.
Why this is correct
EIGRP automatically installs a null0 route for summary addresses to prevent loops.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The route has a metric of 2560000000, indicating a network failure.
Why it's wrong here
A high metric for a null0 route is expected and does not indicate failure.
✗
The hop count of 0 means the route is not reachable.
Why it's wrong here
Hop count of 0 is normal for a directly connected or summary route.
✗
The route is in Active state, indicating a problem.
Why it's wrong here
The state is Passive, which is normal.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a Null0 route or a high metric indicates a failure, when in fact it is a normal and necessary part of EIGRP summarization to prevent black holes.

Detailed technical explanation

How to think about this question

When EIGRP manual summarization is configured, the router creates a local route pointing to Null0 with a composite metric calculated from the summary's component routes. This Null0 route prevents routing loops by discarding packets that match the summary but not a more specific route. The metric of 2560000000/0 is derived from the default bandwidth (100000 Kbit) and delay (100 microseconds) used for the summary, and the hop count of 0 indicates the route is locally generated.

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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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?

EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The route is a local summary route pointing to Null0, which is normal for EIGRP summarization. — The output shows a Null0 summary route in EIGRP, which is normal when manual summarization is configured. The route is in Passive state with a composite metric of 2560000000/0, indicating it is a local summary route installed to prevent routing loops. This is expected behavior, not a problem.

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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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