hardMultiple ChoiceObjective-mapped
300-410 Practice Question: In a VRF-Lite network, OSPF is configured with a…
In a VRF-Lite network, OSPF is configured with a distribute-list that filters routes from being installed in the routing table. The engineer notices that the distribute-list is working, but the filtered routes are still being advertised to OSPF neighbors. 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
✓
The distribute-list is applied inbound, which only prevents the route from being installed in the RIB, but the route remains in the OSPF LSDB and is advertised to neighbors.
The OSPF distribute-list in (or out) filters routes from being installed in the routing table (RIB) but does not affect the OSPF link-state database (LSDB) or the advertisement of routes. To filter routes from being advertised, the distribute-list must be applied outbound, or an area filter (using 'area range' with 'not-advertise') must be used. This is a common misconception: distribute-list in only affects local RIB installation, not LSDB propagation.
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 inbound, which only prevents the route from being installed in the RIB, but the route remains in the OSPF LSDB and is advertised to neighbors.
Why this is correct
Distribute-list in filters routes from the RIB, not the LSDB. OSPF advertises based on the LSDB, so the route is still advertised.
- ✗
The distribute-list is applied outbound, but the neighbor has a higher priority, so the route is still advertised.
Why it's wrong here
Outbound distribute-list filters advertisements, so the route would not be sent.
- ✗
The OSPF process has the 'database-filter all out' command configured, which overrides the distribute-list.
Why it's wrong here
Database-filter would block all advertisements, not just the filtered ones.
- ✗
The distribute-list uses an ACL that does not match the route correctly, so the route is not filtered.
Why it's wrong here
The question states the distribute-list is working (routes are not in RIB), so the ACL is correct.
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
Introduction to ENARSI Exam and Network Fundamentals
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.
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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