hardMultiple Choice
300-410 Practice Question: An engineer configures a route map to filter OSPF…
An engineer configures a route map to filter OSPF routes using a distribute-list in OSPF process 1. The distribute-list references a prefix-list that permits only the 10.0.0.0/8 network. After applying the distribute-list in, the engineer notices that the OSPF neighbor state remains stuck in EXSTART/EXCHANGE. Which is the most likely explanation?
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
✓
There is an MTU mismatch between the OSPF neighbors, preventing the exchange of Database Description packets.
OSPF distribute-list in only filters routes in the routing table, not LSAs. It does not affect the exchange of LSAs during adjacency formation. The adjacency stall is unrelated to the distribute-list; the issue is likely an MTU mismatch between the interfaces.
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 incorrectly; it should be applied out instead of in.
Why it's wrong here
Distribute-lists filter the local OSPF routing table, not link-state advertisements, so they cannot stall adjacency formation; EXSTART/EXCHANGE failures stem from MTU mismatch or duplicate router IDs. Applying the list outbound is tempting because outbound filtering controls what routes are advertised to neighbours, which is valid for influencing their routing tables, not for this symptom.
- ✗
The distribute-list filters LSAs during the exchange, causing the neighbor to be stuck.
Why it's wrong here
Distribute-lists operate on routes installed in the local RIB after SPF, never on LSAs traversing the adjacency, so they cannot block database exchange; EXSTART/EXCHANGE stalls indicate MTU mismatch or duplicate router IDs. Filtering LSAs is tempting because area or ASBR filters do suppress type 3, 5 and 7 LSAs, which is the correct tool for controlling inter-area or external prefix propagation.
- ✓
There is an MTU mismatch between the OSPF neighbors, preventing the exchange of Database Description packets.
Why this is correct
OSPF neighbours exchange Database Description packets during EXSTART, and these packets carry the interface MTU. A mismatch causes the routers to reject each other's DBD packets, so the adjacency stalls in EXSTART/EXCHANGE regardless of the distribute-list filtering.
- ✗
The prefix-list is misconfigured; it should permit 10.0.0.0/8 with a ge 24 operator.
Why it's wrong here
EXSTART/EXCHANGE indicates an MTU mismatch or stuck database exchange, not a prefix-list problem; distribute-list in filters LSAs after adjacency forms. The ge 24 operator would match more-specific /24s, which is a filtering concern unrelated to neighbour state.
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.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
Policy Based Routing
Policy Based Routing (PBR) is a technology that allows network traffic to be forwarded based on user-defined policies rather than just the destination IP address in the routing table.
Key term
ACL for Routing Protocols
An ACL for Routing Protocols is a set of rules used to filter routing updates between routers, controlling which network routes are advertised or accepted to improve security and stability.
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Written by Johnson Ajibi, MSc IT Security
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