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300-410 Practice Question: An engineer configures an IPv4 ACL on a router's…
An engineer configures an IPv4 ACL on a router's interface to permit only HTTP traffic (TCP port 80) from a specific subnet. The ACL is applied inbound. After applying, the router's web interface (HTTPS) becomes unreachable from the same subnet. What is the most likely explanation?
⚠ Common exam trap
Cisco often tests the implicit deny any behavior of ACLs and the fact that management protocols (like HTTPS, SSH, SNMP) use different ports than the permitted traffic, causing candidates to overlook the need to explicitly permit those ports.
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 denies HTTPS traffic because it is not explicitly permitted.
The ACL is applied inbound on the router interface and only permits TCP port 80 (HTTP). HTTPS uses TCP port 443, which is not explicitly permitted. Since IPv4 ACLs end with an implicit deny any, all traffic not matching a permit statement, including HTTPS, is denied. This causes the router's web interface (HTTPS) to become unreachable from the subnet.
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 denies HTTPS traffic because it is not explicitly permitted.
Why this is correct
The ACL only permits HTTP; HTTPS (port 443) is denied by the implicit deny, blocking management access.
- ✗
The ACL is applied outbound, filtering traffic to the web server.
Why it's wrong here
Outbound ACL would affect outgoing traffic, not incoming management.
- ✗
The router's web server uses HTTP, not HTTPS.
Why it's wrong here
Modern routers use HTTPS by default; HTTP is often disabled.
- ✗
The ACL is blocking TCP port 80 due to a typo.
Why it's wrong here
The ACL permits port 80, so HTTP should work.
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