hardMultiple ChoiceObjective-mapped
300-410 OSPF distribute-list outbound Practice Question
An engineer configures BFD on an OSPF link and also applies a distribute-list outbound under the OSPF process. The BFD session is up, but OSPF routes are not being advertised to the neighbor. The engineer verifies that the distribute-list is correctly configured. What is the most likely cause?
⚠ Common exam trap
The trap is that distribute-list outbound under OSPF does not filter intra-area or inter-area LSAs—only redistributed routes. However, on an ABR with 'area range', the summary LSA can be filtered.
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 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
OSPF distribute-list outbound only filters routes that are being redistributed into OSPF, not intra-area or inter-area LSAs. On an ABR, the 'area range' command generates a summary LSA (type 3), which can be filtered by a distribute-list outbound if it references that specific summary route. In this scenario, the distribute-list is correctly configured to deny the network, and the ABR summarizes that network via 'area range', causing the summary LSA to be filtered and not advertised to the neighbor. BFD does not affect this behavior.
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 outbound, but OSPF distribute-lists only filter routes in the routing table, not LSAs, so the routes are still advertised.
Why it's wrong here
This is incorrect because although distribute-list outbound does filter routes from being installed in the routing table, it does not prevent LSA advertisement for intra-area and inter-area routes. However, on an ABR, if an 'area range' is configured, the summary LSA can be filtered by the distribute-list, which would prevent advertisement—contradicting the statement that routes are still advertised.
- ✗
The BFD session is flapping due to a mismatch in the 'bfd interval' command, causing OSPF to withdraw routes.
Why it's wrong here
The BFD session is up per the scenario, so flapping is unlikely. BFD mismatch would cause the session to go down, not just withdraw routes while the session is up.
- ✗
The distribute-list references an ACL that denies the network, but OSPF uses type 3 LSAs that are not affected by ACLs.
Why it's wrong here
OSPF distribute-lists can reference ACLs to filter specific routes, but ACLs do filter based on network addresses. However, the statement that type 3 LSAs are not affected by ACLs is false; ACLs can match the network in the summary LSA. But the key point is that the distribute-list outbound on an ABR can filter the summary LSA generated by 'area range'.
- ✓
The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
Why this is correct
This is correct. When an ABR has an 'area range' command summarizing a network, it creates a type 3 LSA for that summary. If a distribute-list outbound is applied that denies that summary network, the ABR will not advertise the type 3 LSA to neighbors in other areas. This explains why routes are not being advertised despite the BFD session being up.
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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