- A
The ACL permits only SSH, but the implicit deny blocks all other traffic, including EIGRP hellos, causing the neighbor relationship to fail and making the loopback unreachable.
Without explicit permit for routing protocols, the ACL blocks them, causing routing failure.
- B
The ACL is applied inbound, so it filters traffic entering R1; SSH traffic from R2 to R1's loopback is outbound from R2, so it is not affected.
Why wrong: The ACL is inbound on R1, so it filters traffic entering R1. SSH traffic from R2 to R1's loopback enters R1 via GigabitEthernet0/0, so it is filtered.
- C
R2's SSH client uses a source port that is not TCP 22.
Why wrong: SSH uses destination port 22, not source port.
- D
R1's loopback interface has a separate ACL that blocks SSH.
Why wrong: No ACL on loopback is mentioned.
ACL Causing EIGRP Adjacency Failure
This 300-410 practice question tests your understanding of eigrp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network uses ACLs to filter traffic. Router R1 has the following ACL applied to interface GigabitEthernet0/0 in the inbound direction: access-list 100 permit tcp any any eq 22. Router R2, connected to R1, cannot SSH to R1's loopback address. R1 shows: 'show ip interface GigabitEthernet0/0' indicates the ACL is applied. What is the root cause?
Quick Answer
The answer is the ACL blocking EIGRP hellos, which prevents the neighbor adjacency from forming between R1 and R2. The applied ACL permits only TCP port 22 for SSH traffic, but its implicit deny at the end silently drops all other packets, including EIGRP multicast hellos on 224.0.0.10. Without those hellos, the EIGRP neighbor relationship fails, making R2’s loopback unreachable even if SSH packets themselves could theoretically pass. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that ACLs applied to interfaces filter control plane traffic like routing protocol packets, not just user data—a common trap is to focus only on the permitted SSH and forget the implicit deny. Remember the memory tip: “ACLs don’t care about routing; they block everything not explicitly allowed.”
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 ACL permits only SSH, but the implicit deny blocks all other traffic, including EIGRP hellos, causing the neighbor relationship to fail and making the loopback unreachable.
The ACL applied inbound on R1's GigabitEthernet0/0 permits only SSH (TCP port 22) and implicitly denies all other traffic. EIGRP uses multicast IP 224.0.0.10 and protocol number 88, which is not TCP and not permitted by the ACL. Without EIGRP hellos, the neighbor relationship between R1 and R2 fails, making R1's loopback unreachable from R2 because EIGRP cannot install the route.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 ACL permits only SSH, but the implicit deny blocks all other traffic, including EIGRP hellos, causing the neighbor relationship to fail and making the loopback unreachable.
Why this is correct
Without explicit permit for routing protocols, the ACL blocks them, causing routing failure.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The ACL is applied inbound, so it filters traffic entering R1; SSH traffic from R2 to R1's loopback is outbound from R2, so it is not affected.
- ✗
R2's SSH client uses a source port that is not TCP 22.
Why it's wrong here
SSH uses destination port 22, not source port.
- ✗
R1's loopback interface has a separate ACL that blocks SSH.
Why it's wrong here
No ACL on loopback is mentioned.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the implicit deny at the end of every ACL and how it can break routing protocols (like EIGRP, OSPF, or BGP) when the ACL does not explicitly permit the protocol's traffic, leading candidates to overlook the impact on control plane traffic.
Detailed technical explanation
How to think about this question
EIGRP uses multicast address 224.0.0.10 with protocol number 88 in the IP header, not TCP or UDP. An ACL with 'permit tcp any any eq 22' only matches TCP packets with destination port 22; all other IP traffic, including EIGRP hellos, is implicitly denied. In a real-world scenario, this misconfiguration can cause routing protocol adjacency loss, making remote loopbacks unreachable even though the SSH traffic itself is permitted.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL permits only SSH, but the implicit deny blocks all other traffic, including EIGRP hellos, causing the neighbor relationship to fail and making the loopback unreachable. — The ACL applied inbound on R1's GigabitEthernet0/0 permits only SSH (TCP port 22) and implicitly denies all other traffic. EIGRP uses multicast IP 224.0.0.10 and protocol number 88, which is not TCP and not permitted by the ACL. Without EIGRP hellos, the neighbor relationship between R1 and R2 fails, making R1's loopback unreachable from R2 because EIGRP cannot install the route.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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