hardMultiple Choice
300-410 Practice Question: An engineer configures mutual redistribution…
An engineer configures mutual redistribution between OSPF and EIGRP. After a few minutes, routing loops occur. The engineer did not use route tagging. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that routing loops in mutual redistribution are caused by administrative distance or metric issues, when in fact the core problem is the lack of route tagging to prevent re-redistribution of routes back into their original protocol.
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
✓
Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle.
Mutual redistribution without route tagging creates a feedback loop: OSPF redistributes EIGRP-learned routes back into OSPF, and EIGRP redistributes OSPF-learned routes back into EIGRP. Each protocol re-advertises the other's routes, causing them to be learned and re-injected repeatedly, which leads to routing loops. Route tagging (e.g., using a route-map to set a tag) is the standard method to prevent such cycles by filtering redistributed routes that already originated from the other protocol.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Routes redistributed from OSPF into EIGRP have a higher administrative distance than the original OSPF routes, causing them to be preferred.
Why it's wrong here
Administrative distance is used to select the best path when the exact same prefix is learned from multiple routing sources. An EIGRP external route (AD 170) is actually less preferred than an OSPF internal route (AD 110), so this statement is factually reversed, but even if one route were preferred, that only determines the forwarding path. It does not stop either protocol from re-advertising the other's routes; the loop is caused by unconditional re-redistribution, not by AD values.
- ✗
Redistributed routes retain their original metric, which can cause them to be preferred over the original route.
Why it's wrong here
Redistributed routes do not retain their original metric; each protocol assigns a new seed metric (such as an OSPF cost or EIGRP composite metric) using defaults or a route-map. Metrics are only comparable within the same routing protocol, so a lower metric on a redistributed route would merely make it the preferred path inside that protocol. This is normal path selection and does not explain the continuous re-injection cycle, which stems from the lack of route filtering.
- ✓
Without route tagging, OSPF redistributes EIGRP routes back into OSPF, and EIGRP redistributes OSPF routes back into EIGRP, creating a cycle.
Why this is correct
Without route tagging, OSPF and EIGRP cannot distinguish routes that originated in their own domain from routes that were redistributed from the other protocol. OSPF redistributes an EIGRP route (which may already contain OSPF-derived prefixes) back into OSPF, and EIGRP does the same in reverse, so each protocol repeatedly re-injects the other's routes. This cycle creates route flapping and potential loops; a route-map that tags and then denies re-redistribution of tagged routes prevents the feedback.
- ✗
The seed metric for redistribution is not set, causing the redistributed routes to be rejected.
Why it's wrong here
A missing seed metric for redistribution would cause the target protocol to reject those routes—EIGRP specifically requires metric components, otherwise it does not advertise them. If routes are rejected, they cannot be re-redistributed, so no loop occurs. The loop requires successful redistribution of routes in both directions, and the missing piece is route filtering, not the seed metric.
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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