300-410 Practice Question: Management traffic to a router's loopback…

Correct answer & explanation

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.