hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting a route filtering problem with…
A network engineer is troubleshooting a route filtering problem with prefix-lists. Router R6 is using a prefix-list to filter routes from a BGP neighbor. The prefix-list is configured to permit only 192.168.0.0/16 and 192.168.1.0/24, but routes with prefix 192.168.2.0/24 are also being accepted. The engineer checks the prefix-list configuration and sees only two permit statements. What is the most likely cause?
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 prefix-list is not applied to the BGP neighbor; the neighbor is using a different filter or no filter.
Prefix-lists have an implicit deny at the end, but if the prefix-list is not applied correctly (e.g., to the neighbor inbound direction), or if the prefix-list has a 'ge' or 'le' operator that is too broad, it could permit more than intended. However, the most common issue is that the prefix-list is not applied at all, or the neighbor is using a different filter.
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 prefix-list is not applied to the BGP neighbor; the neighbor is using a different filter or no filter.
Why this is correct
Correct because if the prefix-list is not applied, no filtering occurs.
- ✗
The prefix-list has an implicit permit at the end for all routes.
Why it's wrong here
Incorrect because prefix-lists have an implicit deny at the end.
- ✗
The prefix-list is using 'ge 24' which permits any prefix with a mask >= 24, including 192.168.2.0/24.
Why it's wrong here
Incorrect because the scenario says only two permit statements without ge/le.
- ✗
The BGP neighbor is configured with 'soft-reconfiguration inbound' which overrides prefix-list filtering.
Why it's wrong here
Incorrect because soft-reconfiguration stores updates but does not override filtering.
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Introduction to ENARSI Exam and Network Fundamentals
Key term
BGP Prefix Filtering
BGP Prefix Filtering is the practice of controlling which network routes (prefixes) a router accepts or advertises to its BGP neighbors, preventing unwanted or harmful routes from spreading across the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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