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300-410 Practice Question: Configures mutual redistribution between EIGRP…
A network engineer configures mutual redistribution between EIGRP and OSPF on a DMVPN hub router. The EIGRP domain includes the DMVPN tunnel network, and OSPF includes a corporate backbone. Unexpectedly, after a few minutes, the routing table on the hub shows oscillating routes between EIGRP and OSPF for the same prefix, causing intermittent connectivity. 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 mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD.
Mutual redistribution without route tagging or filtering can cause routing loops. When a route is redistributed from EIGRP into OSPF, it is then redistributed back into EIGRP with a different administrative distance (AD). Since OSPF has AD 110 and EIGRP has AD 90 (internal) or 170 (external), the redistributed route may be preferred over the original, causing a feedback loop. The corner case is that the default AD values cause the redistributed route to be installed, and then the router advertises it back, leading to instability. The fix is to use route tags or distribute-lists to prevent re-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.
- ✓
The mutual redistribution creates a routing loop because the redistributed routes are re-injected into the original protocol without proper filtering, causing the hub to prefer the redistributed route with a lower AD.
Why this is correct
Without route tagging, a route redistributed from EIGRP into OSPF (AD 110) and then back into EIGRP (AD 170) may be preferred over the original EIGRP internal route (AD 90) if the AD is misconfigured, but typically the original internal route has lower AD. However, if the route is external in EIGRP, the AD is 170, so the OSPF route (110) is preferred, causing a loop.
- ✗
The DMVPN tunnel interface is not included in the OSPF process, causing the redistributed routes to have an incorrect next-hop that points to the tunnel interface.
Why it's wrong here
The tunnel interface being in EIGRP does not directly cause the oscillation; the issue is the redistribution loop.
- ✗
The EIGRP and OSPF processes are using different metric styles, causing the redistributed routes to have infinite metrics and be ignored.
Why it's wrong here
If metrics are not set correctly, redistribution may fail, but the symptom is oscillation, not failure.
- ✗
The hub router's routing table is overloaded due to the DMVPN tunnel being a multipoint interface, causing route flapping.
Why it's wrong here
Route flapping due to interface issues is possible, but the described symptom of oscillation between protocols indicates a redistribution loop.
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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