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BGP Troubleshooting →hardMultiple ChoiceObjective-mapped

300-410 BGP Troubleshooting Practice Question

This 300-410 practice question tests your understanding of bgp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.

R1 and R2 are eBGP peers. R1 advertises a prefix 172.16.1.0/24 with AS_PATH 65001 65002. R2 receives the prefix and its BGP table shows AS_PATH 65001 65002. R2 has a route-map applied inbound that sets local-preference 150 for routes with AS_PATH containing 65002. R2 also has another eBGP peer R3 that advertises the same prefix with AS_PATH 65001 65003. R2's BGP best path selection chooses the path via R3 because of shorter AS_PATH length (2 vs 3). However, the network engineer expects the path via R1 to be preferred due to the higher local-preference. What is the root cause?

A
The route-map applied inbound on R2 from R1 has a match ip address prefix-list that does not include 172.16.1.0/24, so local-preference is not set.
If the match condition fails, the set commands are not executed, and the default local-preference (100) applies.
B
AS_PATH length is always preferred over local-preference in BGP best path selection.
Why wrong: Local-preference is evaluated before AS_PATH length.
C
The route-map should be applied outbound on R1 instead.
Why wrong: Local-preference is typically set inbound on the receiving router.
D
R2's BGP table shows the local-preference as 150 for the path via R1, but the best path selection ignores it due to a bug.
Why wrong: If local-preference were 150, it would be preferred. The show output would confirm the actual local-preference.

Full breakdown with real-world context →

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-map applied inbound on R2 from R1 has a match ip address prefix-list that does not include 172.16.1.0/24, so local-preference is not set.

Local-preference is evaluated before AS_PATH length in BGP best path selection. If the local-preference is set to 150 on the path via R1, it should be preferred over the path via R3 (which likely has default local-preference 100). The fact that R2 chooses R3 indicates that the local-preference was not applied correctly. The root cause is that the route-map is applied inbound on R2 from R1, but the route-map may have a match clause that does not match the prefix, or the set local-preference command is missing. In this scenario, the route-map is applied but the set local-preference is not executed because the match condition fails (e.g., match ip address prefix-list that does not include 172.16.1.0/24).

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-map applied inbound on R2 from R1 has a match ip address prefix-list that does not include 172.16.1.0/24, so local-preference is not set.
Why this is correct
If the match condition fails, the set commands are not executed, and the default local-preference (100) applies.
Related concept
Read the scenario before looking for a memorised answer.
✗
AS_PATH length is always preferred over local-preference in BGP best path selection.
Why it's wrong here
Local-preference is evaluated before AS_PATH length.
✗
The route-map should be applied outbound on R1 instead.
Why it's wrong here
Local-preference is typically set inbound on the receiving router.
✗
R2's BGP table shows the local-preference as 150 for the path via R1, but the best path selection ignores it due to a bug.
Why it's wrong here
If local-preference were 150, it would be preferred. The show output would confirm the actual local-preference.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Trap categories for this question

Command / output trap
If local-preference were 150, it would be preferred. The show output would confirm the actual local-preference.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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.

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

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

What is the correct answer to this question?

The correct answer is: The route-map applied inbound on R2 from R1 has a match ip address prefix-list that does not include 172.16.1.0/24, so local-preference is not set. — Local-preference is evaluated before AS_PATH length in BGP best path selection. If the local-preference is set to 150 on the path via R1, it should be preferred over the path via R3 (which likely has default local-preference 100). The fact that R2 chooses R3 indicates that the local-preference was not applied correctly. The root cause is that the route-map is applied inbound on R2 from R1, but the route-map may have a match clause that does not match the prefix, or the set local-preference command is missing. In this scenario, the route-map is applied but the set local-preference is not executed because the match condition fails (e.g., match ip address prefix-list that does not include 172.16.1.0/24).

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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