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 using OSPF as the routing protocol. The OSPF neighbor forms, but routes are not being exchanged. The engineer verifies that the IPsec tunnel is up and that OSPF packets are being encrypted. The OSPF network type on the tunnel interface is set to broadcast. What is the most likely explanation for the missing routes?
⚠ Common exam trap
Cisco often tests the misconception that an IPsec tunnel being up guarantees all traffic is encrypted, but the trap here is that OSPF multicast packets are not matched by the default crypto ACL, causing OSPF to fail silently.
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 crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption.
OSPF hello packets are sent to the multicast address 224.0.0.5, but IPsec crypto maps by default only encrypt unicast traffic. Since the crypto map does not match multicast packets, OSPF hellos are dropped before encryption, preventing OSPF neighbor adjacency from forming even though the IPsec tunnel is up and other packets are encrypted.
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 OSPF network type broadcast requires a DR/BDR election, but the tunnel is point-to-point, so the DR election fails and routes are not exchanged.
Why it's wrong here
Broadcast network type can work on a tunnel if both routers are configured correctly; DR election will occur, but it does not prevent route exchange.
- ✓
The IPsec crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption.
Why this is correct
If the crypto ACL only permits unicast traffic (e.g., 'permit ip host A host B'), multicast OSPF packets are not matched and are sent in clear text or dropped, depending on the configuration. This is a common edge case where the interesting traffic definition does not include multicast.
- ✗
The OSPF hello and dead intervals are mismatched, preventing the neighbor from forming.
Why it's wrong here
The question states the neighbor forms, so timers are not the issue.
- ✗
The IPsec tunnel is using transport mode, which does not support multicast traffic.
Why it's wrong here
Transport mode can encapsulate multicast traffic if the tunnel is between two hosts; it is not a limitation.
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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JA
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.