hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures an IPsec site-to-site VPN…
An engineer configures an IPsec site-to-site VPN between two routers running EIGRP. The EIGRP neighbor forms, but routes are not being exchanged. The engineer notices that the EIGRP neighbor is stuck in active state for certain routes. What is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that EIGRP neighbor formation implies full route exchange, but the trap here is that a one-way crypto ACL permits neighbor formation (since hellos are multicast and may be permitted) but blocks query/reply unicast traffic, causing SIA routes.
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 IPsec ACL is permitting EIGRP packets (protocol 88) only in one direction, so queries are sent but replies are dropped by the remote router's crypto map.
When EIGRP neighbors form but routes are not exchanged and the neighbor is stuck in active (SIA) state, it indicates that EIGRP queries are being sent but replies are not received. In an IPsec VPN, if the crypto ACL permits EIGRP (protocol 88) only in one direction, queries from one router are encrypted and sent, but the remote router's crypto map does not match the reply packets, so they are dropped. This prevents the EIGRP query/reply process from completing, causing routes to remain in active state and not be exchanged.
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 EIGRP hello packets are being encrypted but the reply is not, causing asymmetric routing.
Why it's wrong here
Asymmetric routing would not cause SIA; EIGRP uses reliable transport for queries and replies, and unidirectional packet loss is the key issue.
- ✓
The IPsec ACL is permitting EIGRP packets (protocol 88) only in one direction, so queries are sent but replies are dropped by the remote router's crypto map.
Why this is correct
If the crypto ACL on one router permits only certain traffic (e.g., TCP/179 for BGP) but not EIGRP, EIGRP packets may be dropped. If the other router's ACL permits EIGRP, the neighbor forms partially, but queries may not be replied to, causing SIA.
- ✗
The EIGRP K-values are mismatched between the two routers.
Why it's wrong here
Mismatched K-values prevent neighbor formation entirely, not causing SIA.
- ✗
The IPsec tunnel is using aggressive mode, which does not support multicast traffic.
Why it's wrong here
Aggressive mode affects IKE phase 1, not the encapsulation of multicast traffic; EIGRP uses multicast 224.0.0.10, which is supported in tunnel mode.
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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.