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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?

Clue words in this question

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

Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.

Question 1hardmultiple choice

Open the full BGP breakdown →

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: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

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

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

Real-world example

How this comes up in practice

A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

What to study next

Got this wrong? Here's your next step.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 300-410 question test?

BGP Troubleshooting — This question tests BGP Troubleshooting — Static NAT maps one inside address to one outside address..

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?

Are there clue words in this question I should notice?

Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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