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300-410 Practice Question: R1 and R2 are connected via an IPsec VPN tunnel
R1 and R2 are connected via an IPsec VPN tunnel. They are running EIGRP over the tunnel. R1's show ip eigrp neighbors shows R2 as up, but R1's show ip eigrp topology shows a route from R2 as 'stuck-in-active' (SIA). R1's show ip eigrp traffic shows queries being sent but no replies. What is the root cause?
⚠ Common exam trap
Cisco often tests the misconception that a neighbor being 'up' in 'show ip eigrp neighbors' guarantees full EIGRP communication, when in fact the neighbor state can remain up due to unicast keepalives or a stale hold timer while multicast-based queries are blocked by IPsec ACLs.
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 tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped.
EIGRP uses multicast address 224.0.0.10 for neighbor discovery and route updates. If the IPsec VPN tunnel is misconfigured to not encrypt multicast traffic (e.g., using an ACL that only matches unicast or specific protocols), the EIGRP queries sent as multicast packets will be dropped by the IPsec process or the remote router, preventing replies. This results in the neighbor appearing up (due to unicast keepalives or a stale state) but routes stuck-in-active because queries are never answered.
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 IPsec tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped.
Why this is correct
EIGRP queries are sent to multicast address 224.0.0.10. If the crypto map does not protect multicast traffic, queries may be dropped, leading to SIA.
- ✗
EIGRP split-horizon is enabled on the tunnel interface.
Why it's wrong here
Split-horizon affects route advertisement, not query/reply process.
- ✗
The EIGRP active timer is set too low.
Why it's wrong here
A low active timer would cause SIA faster, but the root cause is missing replies.
- ✗
R2 has a passive interface for the tunnel.
Why it's wrong here
Passive interface would prevent R2 from sending hellos, but neighbor is up.
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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