300-410 Layer 3 Technologies Practice Question
A network administrator is troubleshooting an EIGRP adjacency that is stuck in the ACTIVE state. The adjacency is between two routers, R1 and R2, on a point-to-point link. The administrator notices that R1 is sending queries but not receiving replies. Which of the following is the most likely cause?
⚠ Common exam trap
The trap here is assuming that any EIGRP adjacency issue is due to mismatched parameters like AS number or K values, but those would prevent adjacency formation, not cause a stuck-in-active state.
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
✓
A stuck-in-active condition is caused by a router not receiving replies to its queries, often due to a unidirectional link or a neighbor that is unable to respond.
EIGRP routers enter the ACTIVE state when they lose a route and must query neighbors for an alternative path. If a neighbor does not respond to queries, the route can become stuck-in-active. This is often due to unidirectional link failures, high CPU on the neighbor, or packet filters that allow hellos but block other EIGRP packets. The symptom of queries being sent but no replies received points directly to a communication issue in one direction.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A stuck-in-active condition is caused by a router not receiving replies to its queries, often due to a unidirectional link or a neighbor that is unable to respond.
Why this is correct
The ACTIVE state in EIGRP indicates that a route has been lost and the router is actively querying its neighbors for an alternative path. If replies are not received within the active timer (default 3 minutes), the route becomes stuck-in-active. Common causes include unidirectional links, packet loss, or a neighbor that is too busy to respond. In this scenario, R1 is sending queries but not receiving replies, which aligns with a unidirectional link or a neighbor issue.
- ✗
The autonomous system number is misconfigured on R1.
Why it's wrong here
A mismatched autonomous system number would prevent the routers from forming an adjacency at all, as EIGRP requires matching AS numbers for neighbor establishment. The adjacency would not reach the ACTIVE state; it would not form. Therefore, this is not the cause of the ACTIVE state.
- ✗
An access list is blocking EIGRP multicast packets on R2's interface.
Why it's wrong here
If an access list were blocking EIGRP multicast packets, the initial adjacency would not form because EIGRP hellos are multicast to 224.0.0.10. The adjacency would not reach the ACTIVE state; it would be stuck in INIT or not form at all. Therefore, this is not the most likely cause of an adjacency stuck in ACTIVE.
- ✗
The K values for EIGRP metric calculation are mismatched between R1 and R2.
Why it's wrong here
Mismatched K values would prevent the routers from forming an adjacency in the first place, as they are part of the EIGRP hello packet and must match. If they were mismatched, the routers would not become neighbors, and the adjacency would not reach the ACTIVE state. The ACTIVE state indicates that the router has lost a route and is querying neighbors, so the adjacency must have been established previously.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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