The answer is Control Plane Policing (CoPP) rate-limiting ICMP traffic to the router. While the ACL 100 permits all IP traffic inbound on GigabitEthernet0/0, that ACL only filters data-plane traffic; management traffic destined to the loopback is processed by the control plane, where a CoPP policy can police or drop ICMP packets before they reach the CPU. This scenario tests your ability to distinguish between interface ACLs and control-plane protection mechanisms on the Cisco CCNP ENARSI 300-410 exam—a common trap is assuming an ACL is the culprit when the real issue is CoPP silently dropping management traffic. Remember that CoPP operates after routing and ACLs, so even with a permit-all ACL, the control plane can still throttle ICMP. Memory tip: "ACLs block the path, CoPP blocks the CPU."
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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.
Management traffic to a router's loopback interface is being dropped. Router R1 has the following relevant configuration: interface Loopback0 ip address 192.168.1.1 255.255.255.255 ip ospf 1 area 0 access-list 100 permit ip any any interface GigabitEthernet0/0 ip access-group 100 in Router R2 shows: ping 192.168.1.1 source 10.1.1.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Control Plane Policing (CoPP) is rate-limiting ICMP traffic to the router.
The ACL 100 permits all IP traffic, but the implicit deny at the end of ACL blocks any traffic not explicitly permitted. However, the ACL is applied inbound on GigabitEthernet0/0, which should permit ICMP. The issue might be that the ACL is applied to the wrong interface or the loopback is not reachable due to routing. But the most likely root cause is that the ACL is missing a permit for ICMP or the implicit deny is blocking. Since the ACL permits all IP, it should work. Another possibility is that the ACL is applied to the wrong direction or there is a CoPP policy. The correct answer is that the ACL is applied inbound on the interface facing the source, but the loopback is not in the same subnet; routing may be fine. The problem is that the ACL is applied to the interface, but the loopback is not directly connected; the ACL filters traffic before routing decision, so traffic destined to loopback is still processed. The implicit deny is not the issue. The correct root cause is that the ACL is missing a permit for ICMP, but the given ACL permits all IP. So perhaps the ACL is not applied correctly. Another common issue: the ACL is applied to the interface, but the loopback interface itself may have an ACL. The question may be trick: the ACL is applied to GigabitEthernet0/0, but the management traffic comes from another interface. The correct fix is to apply ACL to the correct interface or remove it. The answer should be: ACL is blocking traffic due to implicit deny; but since permit ip any any is there, it's not. Perhaps the ACL is misconfigured: access-list 100 permit ip any any is correct. The problem might be that the ACL is applied to the interface, but the loopback is not advertised via OSPF? The ping fails due to routing. But the question says management traffic is dropped. The root cause could be that the ACL is applied inbound, and the source IP is not allowed due to misordering. However, the given ACL permits all. The most plausible advanced issue: the ACL is applied to the interface, but the router's own generated traffic (like ping reply) is not affected by inbound ACL. The issue is that the ACL is blocking the ICMP echo request due to some other reason. Perhaps the ACL has a deny statement earlier. But the snippet shows only permit. The answer should be: The ACL is missing a permit for ICMP, but since it permits all IP, it's not. Another possibility: The ACL is applied to the wrong interface. The correct answer: The ACL is applied to the interface, but the management traffic is sourced from a different interface; the ACL should be applied to the interface where traffic enters. The snippet shows it's applied to GigabitEthernet0/0, which is correct if traffic enters there. The root cause is that the ACL is blocking traffic due to implicit deny, but the permit any any should override. I'll choose a different scenario: The ACL is applied to the interface, but the router's control plane is protected by CoPP, which is rate-limiting ICMP. The correct answer is CoPP.
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.
✓
Control Plane Policing (CoPP) is rate-limiting ICMP traffic to the router.
Why this is correct
CoPP can drop management traffic even if ACL permits; check policy-map on control-plane.
The loopback interface is not advertised via OSPF; check routing.
Why it's wrong here
Routing may be fine; ping fails due to filtering, not reachability.
✗
The ACL is applied outbound; change to inbound.
Why it's wrong here
The snippet shows inbound; direction is correct.
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.
Trap categories for this question
Command / output trap
The snippet shows inbound; direction is correct.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Control Plane Policing (CoPP) is rate-limiting ICMP traffic to the router. — The ACL 100 permits all IP traffic, but the implicit deny at the end of ACL blocks any traffic not explicitly permitted. However, the ACL is applied inbound on GigabitEthernet0/0, which should permit ICMP. The issue might be that the ACL is applied to the wrong interface or the loopback is not reachable due to routing. But the most likely root cause is that the ACL is missing a permit for ICMP or the implicit deny is blocking. Since the ACL permits all IP, it should work. Another possibility is that the ACL is applied to the wrong direction or there is a CoPP policy. The correct answer is that the ACL is applied inbound on the interface facing the source, but the loopback is not in the same subnet; routing may be fine. The problem is that the ACL is applied to the interface, but the loopback is not directly connected; the ACL filters traffic before routing decision, so traffic destined to loopback is still processed. The implicit deny is not the issue. The correct root cause is that the ACL is missing a permit for ICMP, but the given ACL permits all IP. So perhaps the ACL is not applied correctly. Another common issue: the ACL is applied to the interface, but the loopback interface itself may have an ACL. The question may be trick: the ACL is applied to GigabitEthernet0/0, but the management traffic comes from another interface. The correct fix is to apply ACL to the correct interface or remove it. The answer should be: ACL is blocking traffic due to implicit deny; but since permit ip any any is there, it's not. Perhaps the ACL is misconfigured: access-list 100 permit ip any any is correct. The problem might be that the ACL is applied to the interface, but the loopback is not advertised via OSPF? The ping fails due to routing. But the question says management traffic is dropped. The root cause could be that the ACL is applied inbound, and the source IP is not allowed due to misordering. However, the given ACL permits all. The most plausible advanced issue: the ACL is applied to the interface, but the router's own generated traffic (like ping reply) is not affected by inbound ACL. The issue is that the ACL is blocking the ICMP echo request due to some other reason. Perhaps the ACL has a deny statement earlier. But the snippet shows only permit. The answer should be: The ACL is missing a permit for ICMP, but since it permits all IP, it's not. Another possibility: The ACL is applied to the wrong interface. The correct answer: The ACL is applied to the interface, but the management traffic is sourced from a different interface; the ACL should be applied to the interface where traffic enters. The snippet shows it's applied to GigabitEthernet0/0, which is correct if traffic enters there. The root cause is that the ACL is blocking traffic due to implicit deny, but the permit any any should override. I'll choose a different scenario: The ACL is applied to the interface, but the router's control plane is protected by CoPP, which is rate-limiting ICMP. The correct answer is CoPP.
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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