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300-410 Practice Question: Is troubleshooting a route filtering issue using…
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?
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 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'.
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.
- ✗
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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