mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Consider the following partial configuration on a…
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
!
line vty 0 4
transport input telnet password cisco login
What is the effect of this configuration?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
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.
Why it's wrong here
Underscores are allowed in ACL names.
Visual reference
Go deeper
Related to this question
About these practice questions
One of 1,966 original 300-410 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.