Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
ip access-list extended BLOCK-TELNET deny ip any host 10.10.10.10 deny tcp any host 10.10.10.10 eq 23 permit ip any any
Exhibit: An ACL applied inbound on G0/0 is intended to block Telnet to 10.10.10.10 while permitting everything else, but users cannot reach any service on that server. Why?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The ACL must be applied outbound, not inbound
That is not the core logic error shown here.
B
Best answer
The deny ip statement blocks all traffic to the host before the Telnet-specific line is evaluated
The first matching ACE wins.
C
Distractor review
Extended ACLs cannot match TCP port 23
They can.
D
Distractor review
Telnet uses UDP, so the ACE should reference udp
Telnet uses TCP.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that the ACL evaluates all lines before deciding to block or permit traffic. Many candidates mistakenly believe that a specific deny Telnet line will override a broader deny ip statement later in the ACL. However, ACLs stop processing at the first match, so placing a deny ip line before the Telnet-specific deny causes all IP traffic to that host to be blocked. This misunderstanding leads to ACLs that unintentionally block all services, not just Telnet, causing connectivity failures and confusion.
Technical deep dive
How to think about this question
Access Control Lists (ACLs) are fundamental tools in Cisco networking used to filter traffic based on defined criteria such as source/destination IP addresses, protocols, and ports. Extended ACLs allow granular control by matching specific Layer 4 protocols and port numbers, enabling administrators to permit or deny traffic like Telnet (TCP port 23) selectively. When an ACL is applied inbound on an interface, it evaluates packets as they arrive, filtering traffic before routing or switching decisions occur. ACLs are processed sequentially from the top down, and the first matching ACE determines the fate of the packet. If a deny ACE matches, the packet is dropped immediately, and no further ACL entries are checked. This means that if a broad deny statement appears before a specific permit statement, the specific permit will never be reached, causing unintended traffic blocking. Therefore, the order of ACEs is crucial to ensure correct traffic filtering behavior. In the given scenario, the ACL intended to block only Telnet traffic to 10.10.10.10 contains a deny ip statement that blocks all IP traffic to that host before the Telnet-specific deny line is evaluated. This causes all services to be blocked, not just Telnet. The exam trap lies in misunderstanding ACL processing order and the impact of broad deny statements placed before specific ones. Practically, network engineers must carefully order ACL entries to avoid inadvertently blocking legitimate traffic while achieving the desired filtering effect.
KKey Concepts to Remember
- An Access Control List (ACL) processes packets sequentially from top to bottom and stops at the first matching Access Control Entry (ACE).
- A deny ACE in an ACL immediately blocks matching traffic and prevents further ACL entries from being evaluated for that packet.
- Extended ACLs can filter traffic based on Layer 3 IP addresses and Layer 4 protocols and ports, such as TCP port 23 for Telnet.
- Applying an ACL inbound on an interface filters traffic as it enters the router or switch before routing decisions are made.
- The order of ACEs in an ACL is critical; placing a broad deny statement before specific permit statements can block all traffic unintentionally.
- Telnet uses TCP port 23, so ACLs must specify TCP protocol and port 23 to selectively block Telnet traffic.
- Implicit deny at the end of every ACL blocks all traffic not explicitly permitted, so permit statements must be carefully ordered.
- ACLs do not differentiate between services unless explicitly configured with protocol and port numbers in extended ACLs.
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.
Related practice questions
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
CCNA subnetting practice questions
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
CCNA OSPF practice questions
Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
CCNA VLAN practice questions
Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
CCNA STP practice questions
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
CCNA EtherChannel practice questions
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
CCNA ACL practice questions
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
CCNA NAT practice questions
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
CCNA DHCP practice questions
Practise DHCP scopes, relay, leases and troubleshooting.
CCNA show ip route practice questions
Practise routing-table output, longest-prefix match, AD and route selection.
CCNA show interfaces trunk practice questions
Practise trunk verification and VLAN forwarding across switches.
CCNA wireless security practice questions
Practise WLAN security, authentication and wireless architecture concepts.
CCNA IPv6 practice questions
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
An Access Control List (ACL) processes packets sequentially from top to bottom and stops at the first matching Access Control Entry (ACE).
What is the correct answer to this question?
The correct answer is: The deny ip statement blocks all traffic to the host before the Telnet-specific line is evaluated — ACLs are processed top-down. The shown ACL denies all IP traffic to the host before the Telnet-specific ACE could matter, so every protocol toward that server is blocked.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
Discussion
Loading comments…
Sign in to join the discussion.