hardMultiple Choice
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?
⚠ Common exam trap
300-410 often tests whether candidates verify that filters are actually applied; the trap is assuming the prefix-list logic is wrong when the real issue is that it was never attached to the neighbor.
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.
If routes with prefix 192.168.2.0/24 are being accepted despite a prefix-list that only permits 192.168.0.0/16 and 192.168.1.0/24, the most likely cause is that the prefix-list is not actually applied to the BGP neighbor. Without an inbound filter, the neighbor accepts all routes, including 192.168.2.0/24.
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
An unapplied prefix-list cannot filter anything, so the neighbour's inbound or outbound route map, distribute-list or default behaviour governs what is accepted. Since 192.168.2.0/24 passes despite the two permit statements, the filter is evidently not bound to that BGP session, satisfying the stem's requirement that only the listed prefixes be permitted.
- ✗
The prefix-list has an implicit permit at the end for all routes.
Why it's wrong here
Prefix-lists carry an implicit deny, not an implicit permit, so unmatched prefixes are dropped. It is tempting because ACLs and route-maps end with implicit deny too, but confusing the direction leads engineers to suspect a phantom permit; the real cause is a ge or le modifier widening the match.
- ✗
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
Soft-reconfiguration inbound stores unmodified routes for outbound policy changes; it does not bypass inbound prefix-list filtering. It is tempting because it concerns BGP policy behaviour, but it would be relevant when re-applying filters without resetting sessions, not when unwanted prefixes are still accepted.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.