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300-410 Practice Question: An engineer configures mutual redistribution…
An engineer configures mutual redistribution between OSPF and EIGRP on a PE router in an MPLS L3VPN. The engineer does not configure any route tagging or filtering. After a few minutes, the OSPF and EIGRP domains become unstable, with routes flapping and high CPU usage. What 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 mutual redistribution creates a routing loop because routes are redistributed back into the original protocol without any loop-prevention mechanism.
Mutual redistribution without route tagging or filtering can cause a routing loop. When OSPF routes are redistributed into EIGRP, and then those EIGRP routes are redistributed back into OSPF, the same prefixes can be learned from both protocols. Without a route tag or a filter to prevent re-redistribution, the router will continuously re-advertise the same routes, causing route flapping and high CPU. This is a classic edge case in redistribution. The solution is to use route tags and filtering to prevent loops.
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 mutual redistribution creates a routing loop because routes are redistributed back into the original protocol without any loop-prevention mechanism.
Why this is correct
Correct. Without route tagging or filtering, routes can be re-redistributed indefinitely, causing instability.
- ✗
The OSPF and EIGRP administrative distances conflict, causing the router to prefer the wrong route.
Why it's wrong here
Incorrect. AD conflicts can cause suboptimal routing but not the flapping and high CPU described.
- ✗
The 'default-information originate' command is missing, so the redistributed routes are not advertised.
Why it's wrong here
Incorrect. 'default-information originate' is for default routes, not for redistributed routes.
- ✗
The 'subnets' keyword is missing in the OSPF redistribution command, causing only classful routes to be advertised.
Why it's wrong here
Incorrect. Missing 'subnets' would cause incomplete route advertisement, not flapping and high CPU.
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 Neighbor Adjacency and Basics
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.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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