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300-410 Practice Question: An engineer configures mutual redistribution…
An engineer configures mutual redistribution between OSPF and EIGRP on a router. Both protocols have routes for the same prefix. The engineer also applies an inbound ACL on the OSPF interface to deny certain routes from being learned via OSPF. After the ACL is applied, the router still has the prefix in the routing table, but it is learned via EIGRP instead of OSPF. What is the most likely explanation?
⚠ Common exam trap
Cisco often tests the distinction between filtering routing updates via an interface ACL (which blocks the entire OSPF packet, preventing route learning) versus using a distribute-list (which filters specific routes within the routing process), leading candidates to confuse the two mechanisms.
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 ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD.
The inbound ACL on the OSPF interface blocks the OSPF-learned route from being installed in the routing table. Since mutual redistribution is configured, the same prefix is also learned via EIGRP. EIGRP has a default administrative distance (AD) of 90 for internal routes, which is lower than OSPF's AD of 110. Therefore, the router selects the EIGRP route as the best path, and the prefix remains in the routing table but now points to the EIGRP next hop.
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 ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD.
Why this is correct
The ACL prevents OSPF from learning the route, but redistribution allows EIGRP to carry it, and EIGRP's lower AD makes it the best path.
- ✗
The ACL is applied outbound, so it filters OSPF routes being sent to the neighbor.
Why it's wrong here
Outbound ACL would affect outgoing updates, not incoming learning.
- ✗
The router has a static route for the prefix with AD 1.
Why it's wrong here
Static route would override both, but the question does not mention static.
- ✗
The OSPF process has a distribute-list that is filtering the route.
Why it's wrong here
Distribute-list would filter the route from the routing table, but the ACL is the configured filter.
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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