- 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.
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.”
300-410 EIGRP Troubleshooting Practice Question
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?
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 only permits TCP port 22 (SSH) but implicitly denies all other traffic, including routing protocol packets and ICMP. If the SSH traffic is sourced from R2's interface IP, it may be permitted, but if the loopback is not reachable due to routing protocol being blocked, SSH fails.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
- ✗
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: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — OSPF neighbours must agree on key parameters..
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 only permits TCP port 22 (SSH) but implicitly denies all other traffic, including routing protocol packets and ICMP. If the SSH traffic is sourced from R2's interface IP, it may be permitted, but if the loopback is not reachable due to routing protocol being blocked, SSH fails.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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