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350-701 Practice Question: After applying a new extended ACL inbound on an…
After applying a new extended ACL inbound on an interface, users report they can no longer reach a critical server on a different subnet. The ACL permits the server's IP and required ports. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the concept that an inbound ACL filters traffic before the routing decision, and candidates mistakenly focus only on the destination server's IP and ports, forgetting that return traffic must also be explicitly permitted to avoid the implicit deny.
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 is missing a permit for necessary traffic (e.g., return traffic or ARP), triggering the implicit deny.
When an extended ACL is applied inbound on an interface, it filters traffic entering that interface before the routing decision. Even if the ACL permits the destination server's IP and required ports, it must also permit the return traffic (e.g., TCP acknowledgments, ICMP replies) from the server back to the users. If the ACL does not explicitly permit this return traffic, the implicit deny at the end of the ACL will drop it, breaking connectivity. This is the most common cause of connectivity loss after applying an inbound ACL.
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 router has run out of memory for ACL processing.
Why it's wrong here
Unlikely in a typical scenario.
- ✗
The ACL is applied in the outbound direction instead of inbound.
Why it's wrong here
Direction change might cause issues but not typical for access.
- ✗
The ACL is applied to the wrong interface.
Why it's wrong here
Could be, but users report they can't reach server, likely ACL blocking.
- ✓
The ACL is missing a permit for necessary traffic (e.g., return traffic or ARP), triggering the implicit deny.
Why this is correct
Extended ACLs end with implicit deny; missing permit for other traffic blocks communication.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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