The correct answer is that Telnet traffic from the router to remote devices via Serial0/0/0 is blocked, while Telnet sessions initiated to the router itself remain unaffected. This is because the outbound ACL is applied to traffic leaving the interface, meaning any Telnet packets sourced from the router’s own IP address—such as when an administrator issues a `telnet` command from the CLI—are filtered by the `deny tcp any any eq 23` statement. In contrast, inbound management traffic arriving on the VTY lines is not subject to this outbound filter, as ACLs only inspect packets at the interface where they are applied and in the specified direction. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of ACL directionality and the distinction between traffic *originating from* versus *destined to* the router—a common trap is assuming an outbound ACL blocks incoming Telnet to the router. Remember: outbound ACLs control what the router sends out, not what comes in. A useful mnemonic is “Outbound blocks your outbound shouts, inbound blocks what the router’s about.”
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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 partial configuration on a Cisco router:
ip access-list extended BLOCK_TELNET
deny tcp any any eq 23
permit ip any any
!
interface Serial0/0/0
ip access-group BLOCK_TELNET out
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Telnet traffic from the router to remote devices via Serial0/0/0 is blocked; Telnet to the router itself is still allowed.
The ACL BLOCK_TELNET is applied outbound on Serial0/0/0, so it filters traffic leaving that interface. Telnet traffic (TCP port 23) sourced from the router itself (e.g., a user initiating a Telnet session from the router's CLI) is subject to this outbound ACL and is denied. However, Telnet traffic destined to the router (i.e., incoming management sessions to the VTY lines) is not affected because the ACL is not applied inbound on any interface, and the VTY lines have their own authentication and transport input settings.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Telnet traffic from the router to remote devices via Serial0/0/0 is blocked; Telnet to the router itself is still allowed.
Why this is correct
The outbound ACL blocks Telnet leaving the interface, but does not filter traffic destined to the router. VTY lines still accept Telnet.
Related concept
Read the scenario before looking for a memorised answer.
✗
All Telnet traffic to and from the router is blocked.
Why it's wrong here
The ACL is outbound only, so inbound Telnet to the router is not filtered.
✗
The ACL has no effect because it is applied outbound and Telnet is a TCP protocol.
Why it's wrong here
Outbound ACLs can filter TCP traffic; the direction matters, not the protocol type.
✗
The configuration is invalid because the ACL name contains an underscore.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that an outbound ACL on a router's interface will block Telnet sessions to the router itself, when in fact it only affects traffic exiting that interface, not traffic destined to the router's own IP addresses.
Detailed technical explanation
How to think about this question
When an ACL is applied outbound, it filters packets after the routing decision and before they exit the interface. For locally generated traffic (e.g., from the router's own Telnet client), the outbound ACL is evaluated against the outgoing packet. In contrast, traffic destined to the router (like a Telnet session to the VTY lines) is processed by the inbound path on the ingress interface, not by an outbound ACL on a different interface. This distinction is critical for understanding how ACLs affect management plane traffic versus transit traffic.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 300-410 question in full detail.
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Telnet traffic from the router to remote devices via Serial0/0/0 is blocked; Telnet to the router itself is still allowed. — The ACL BLOCK_TELNET is applied outbound on Serial0/0/0, so it filters traffic leaving that interface. Telnet traffic (TCP port 23) sourced from the router itself (e.g., a user initiating a Telnet session from the router's CLI) is subject to this outbound ACL and is denied. However, Telnet traffic destined to the router (i.e., incoming management sessions to the VTY lines) is not affected because the ACL is not applied inbound on any interface, and the VTY lines have their own authentication and transport input settings.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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