300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. 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.
A network engineer is troubleshooting a route filtering issue using distribute-lists under OSPF. Router R8 has a distribute-list out applied to the OSPF process to filter routes being advertised to a specific neighbor. The distribute-list references an ACL that denies a specific prefix, but the prefix is still being advertised to the neighbor. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The distribute-list is applied to the OSPF process, not to the specific neighbor; use 'neighbor distribute-list' instead.
Distribute-lists under OSPF can be applied in or out, but the 'out' direction filters routes being advertised out of the OSPF process to all neighbors. However, if the distribute-list is applied to the OSPF process (not to a specific interface), it filters routes from the routing table into OSPF, not to a specific neighbor. To filter per neighbor, a distribute-list must be applied to the neighbor under the OSPF process using 'neighbor x.x.x.x distribute-list'.
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 distribute-list is applied to the OSPF process, not to the specific neighbor; use 'neighbor distribute-list' instead.
Why this is correct
Correct because per-neighbor filtering requires the distribute-list under the neighbor command.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The ACL is using the wrong wildcard mask; it should deny the exact prefix with 0.0.0.0 mask.
Why it's wrong here
Incorrect because wildcard mask 0.0.0.0 matches only the exact prefix.
✗
The distribute-list is applied inbound instead of outbound.
Why it's wrong here
Incorrect because the scenario says it is applied outbound.
✗
The OSPF process has 'default-information originate' that overrides the distribute-list.
Why it's wrong here
Incorrect because default-information originate does not override distribute-lists.
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.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario says it is applied outbound.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The distribute-list is applied to the OSPF process, not to the specific neighbor; use 'neighbor distribute-list' instead. — Distribute-lists under OSPF can be applied in or out, but the 'out' direction filters routes being advertised out of the OSPF process to all neighbors. However, if the distribute-list is applied to the OSPF process (not to a specific interface), it filters routes from the routing table into OSPF, not to a specific neighbor. To filter per neighbor, a distribute-list must be applied to the neighbor under the OSPF process using 'neighbor x.x.x.x distribute-list'.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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