mediumMultiple Choice
350-401 Is configuring OSPF in a multi-area design Practice Question
A network engineer is configuring OSPF in a multi-area design. The engineer wants to reduce the amount of LSA flooding and the size of the LSDB in area 0. Which OSPF feature should be implemented on the ABR to achieve this goal?
⚠ Common exam trap
Many exam-takers assume area 0 can be made a stub area to reduce LSAs, but Cisco tests the fact that area 0 is a transit area and cannot be a stub, making the filter-list the correct tool for this specific goal.
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
✓
Configure the ABR with an area filter-list to filter type 3 LSAs.
Configuring an area filter-list on the ABR allows the engineer to filter Type 3 summary LSAs entering or leaving area 0. This directly reduces LSA flooding and shrinks the LSDB in area 0 by preventing specific inter-area prefixes from being advertised into the backbone, without altering the area type or requiring additional redistribution.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure area 0 as a stub area.
Why it's wrong here
Configuring area 0 as a stub area is invalid because the OSPF backbone must be able to carry all LSA types, including type 5 external LSAs, to ensure inter-area and external routing capability. Stub areas deliberately block type 5 LSAs (and, in totally stubby areas, type 3 LSAs as well), but the backbone cannot impose such restrictions because it must remain fully transit-capable for all traffic. Attempting this would break OSPF adjacency and inter-area reachability, so it cannot reduce LSDB size in a standardized way.
- ✓
Configure the ABR with an area filter-list to filter type 3 LSAs.
Why this is correct
The area filter-list command on an ABR filters type 3 summary LSAs entering or leaving an area, directly and proactively reducing the number of LSAs in area 0's link-state database. Only type 3 LSAs that describe inter-area prefixes are affected, so selected routes are omitted from the ABR's summary advertisements without disrupting the backbone's transit role or intra-area operations. This is the only option that reduces LSDB size while maintaining correct OSPF design.
- ✗
Configure OSPF database overflow protection.
Why it's wrong here
OSPF database overflow protection, configured with the 'max-lsa' command, only sets a ceiling for the total number of LSAs that a router will accept; it does not filter or suppress the initial flooding of LSAs. When the LSA count exceeds the configured limit, the router enters an overflow state and ceases OSPF operation on the interface, causing temporary outages rather than reducing the LSDB proactively. It is a safety mechanism against database overload, not a method to control inter-area route distribution.
- ✗
Configure the ABR as an ASBR.
Why it's wrong here
Configuring the ABR as an ASBR would introduce external route redistribution, which generates type 5 AS-external LSAs (and type 4 ASBR-summary LSAs) that are flooded into area 0, thereby increasing LSDB size. An ABR that is also an ASBR injects additional route information rather than filtering it, so this action directly contradicts the goal of reducing OSPF overhead. This option would worsen the problem, not mitigate it.
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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