The answer is that the ACL blocks all SSH traffic to the router, but permits other IP traffic. This is correct because the `access-class BLOCK_SSH in` command on the VTY lines applies the extended ACL to incoming Telnet and SSH sessions, filtering traffic before it reaches the router’s management plane. The ACL explicitly denies TCP port 22 (SSH) while permitting all other IP traffic, so any SSH attempt is dropped, but other protocols like Telnet or HTTP remain allowed. On the ENCOR 350-401 exam, this scenario tests your understanding of how `access-class` differs from `access-group`—the former filters VTY line access, not routed traffic. A common trap is assuming the `permit ip any any` statement overrides the deny, but ACLs process top-down, so the deny for SSH is hit first. Remember: VTY lines use `access-class`, interfaces use `access-group`. Memory tip: "Class for lines, Group for interfaces."
350-401 Infrastructure Security Practice Question
This 350-401 practice question tests your understanding of infrastructure security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Consider the following configuration on a Cisco IOS-XE router:
```
ip access-list extended BLOCK_SSH
deny tcp any any eq 22
permit ip any any
!
line vty 0 4
access-class BLOCK_SSH in
```
Which statement is true about this configuration?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL blocks all SSH traffic to the router, but permits other IP traffic.
The access-class applied to the VTY lines filters incoming Telnet/SSH sessions. The ACL denies TCP port 22 (SSH) and permits all other IP traffic. This blocks SSH access to the router.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 blocks all SSH traffic to the router, but permits other IP traffic.
Why this is correct
Correct. The ACL is applied inbound on VTY lines, so SSH (TCP/22) is denied.
Related concept
Standard ACLs match source addresses.
✗
The ACL blocks all traffic to the router because the deny statement is first.
Why it's wrong here
Incorrect. The permit ip any any allows all other traffic.
✗
The ACL only filters traffic going through the router, not destined to it.
Why it's wrong here
Incorrect. An access-class on VTY lines filters traffic destined to the router.
✗
The ACL permits SSH traffic because the permit statement overrides the deny.
Why it's wrong here
Incorrect. The deny is processed first and blocks SSH.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-401 ACL questions on filtering logic and placement.
Infrastructure Security — This question tests Infrastructure Security — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The ACL blocks all SSH traffic to the router, but permits other IP traffic. — The access-class applied to the VTY lines filters incoming Telnet/SSH sessions. The ACL denies TCP port 22 (SSH) and permits all other IP traffic. This blocks SSH access to the router.
What should I do if I get this 350-401 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-401 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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