- A
The prefix-list PL_ALLOW is missing a permit for 10.1.0.0/16 exactly, so it is denied by implicit deny.
The route-map RM_IN has no permit clause for unmatched prefixes, so the implicit deny at the end of the route-map filters 10.1.0.0/16.
- B
The local-preference 200 is too high, causing the route to be suppressed by BGP best-path selection.
Why wrong: Local-preference does not suppress routes; it only influences best path selection.
- C
The neighbor command is missing the soft-reconfiguration inbound option, so the route is not stored.
Why wrong: Soft-reconfiguration is not required for route filtering; it only allows storing pre-filtered routes.
- D
The prefix-list should use ge instead of le to match longer prefixes.
Why wrong: Using le 24 matches prefixes with length 8 to 24, which includes /16. The issue is the route-map implicit deny.
Quick Answer
The answer is the implicit deny at the end of the route-map RM_IN, which filters out 10.1.0.0/16 despite it matching the prefix-list PL_ALLOW’s le 24 range. When a BGP inbound route-map with a prefix-list is applied, only prefixes explicitly permitted by a route-map clause are accepted; any prefix that does not hit a permit clause is dropped by the implicit deny, regardless of whether the prefix-list itself permits it. In this Cisco CCNP ENARSI 300-410 scenario, the route-map only has one permit clause that sets local-preference, so 10.1.0.0/16 is matched by the prefix-list but never reaches a permit clause in the route-map—hence it is denied. This is a common trap: students confuse prefix-list matching with route-map acceptance. The fix is to add a second permit clause without any set actions, or use an explicit permit all. Memory tip: “Route-map permit is the gate; prefix-list match is just the key—if the gate isn’t open, the key doesn’t matter.”
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 large enterprise network is experiencing intermittent loss of reachability to a set of prefixes originated by R2. R1 (AS 65001) and R2 (AS 65002) are eBGP peers. R1 has the following relevant configuration: router bgp 65001, neighbor 10.1.1.2 remote-as 65002, neighbor 10.1.1.2 route-map RM_IN in. The route-map RM_IN has a clause: match ip address prefix-list PL_ALLOW, set local-preference 200. The prefix-list PL_ALLOW permits 10.0.0.0/8 le 24. R2 advertises 10.0.0.0/8 and more specific prefixes including 10.1.0.0/16. R1 shows: BGP table has 10.0.0.0/8 with local-pref 200, but 10.1.0.0/16 is missing. What is the root 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 PL_ALLOW is missing a permit for 10.1.0.0/16 exactly, so it is denied by implicit deny.
The prefix-list PL_ALLOW permits 10.0.0.0/8 le 24, which means it matches prefixes with a length of 8 to 24 bits. However, the more specific prefix 10.1.0.0/16 has a length of 16, which is within the range, but the route-map RM_IN is applied inbound and only sets local-preference for matched prefixes. The missing route is due to the fact that the route-map does not have an explicit permit for unmatched prefixes; by default, an implicit deny applies, causing 10.1.0.0/16 to be filtered out. The correct fix is to add a permit clause without any set actions or use an explicit permit all.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 PL_ALLOW is missing a permit for 10.1.0.0/16 exactly, so it is denied by implicit deny.
Why this is correct
The route-map RM_IN has no permit clause for unmatched prefixes, so the implicit deny at the end of the route-map filters 10.1.0.0/16.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The local-preference 200 is too high, causing the route to be suppressed by BGP best-path selection.
Why it's wrong here
Local-preference does not suppress routes; it only influences best path selection.
- ✗
The neighbor command is missing the soft-reconfiguration inbound option, so the route is not stored.
Why it's wrong here
Soft-reconfiguration is not required for route filtering; it only allows storing pre-filtered routes.
- ✗
The prefix-list should use ge instead of le to match longer prefixes.
Why it's wrong here
Using le 24 matches prefixes with length 8 to 24, which includes /16. The issue is the route-map implicit deny.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
BGP Troubleshooting — This question tests BGP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The prefix-list PL_ALLOW is missing a permit for 10.1.0.0/16 exactly, so it is denied by implicit deny. — The prefix-list PL_ALLOW permits 10.0.0.0/8 le 24, which means it matches prefixes with a length of 8 to 24 bits. However, the more specific prefix 10.1.0.0/16 has a length of 16, which is within the range, but the route-map RM_IN is applied inbound and only sets local-preference for matched prefixes. The missing route is due to the fact that the route-map does not have an explicit permit for unmatched prefixes; by default, an implicit deny applies, causing 10.1.0.0/16 to be filtered out. The correct fix is to add a permit clause without any set actions or use an explicit permit all.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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