Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
Requirement: - Block Telnet from 10.30.30.0/24 to 172.16.9.9 - Allow all other traffic Configured ACL entry: deny tcp 10.30.30.0 0.0.0.255 host 172.16.9.9
Based on the exhibit, why is the ACL blocking more traffic than intended?
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
Best answer
Because the ACL denies all TCP traffic to the server instead of only Telnet.
This is correct because the entry lacks the Telnet destination port qualifier and therefore matches too broadly.
B
Distractor review
Because Telnet uses UDP, not TCP.
This is wrong because Telnet is a TCP-based protocol.
C
Distractor review
Because the ACL should be a standard ACL, not an extended ACL.
This is wrong because protocol and service filtering is precisely why extended ACL logic is needed.
D
Distractor review
Because the host keyword can never be used with TCP statements.
This is wrong because host-specific matches are valid in extended ACLs.
Common exam trap
Common exam trap: answer the scenario, not the keyword
The exam trap here is assuming that denying TCP traffic to a server without specifying the Telnet port will only block Telnet sessions. In reality, the ACL entry without the destination port qualifier matches all TCP traffic to that server, blocking multiple services unintentionally. This mistake often arises from confusing standard ACLs, which filter only by source IP, with extended ACLs that require explicit port numbers for service-specific filtering. Candidates may overlook the need for the 'eq 23' qualifier for Telnet, leading to broader traffic denial and failing the question.
Technical deep dive
How to think about this question
Access Control Lists (ACLs) are fundamental security tools in Cisco networking that control traffic flow by permitting or denying packets based on defined criteria. Extended ACLs provide granular control by allowing matches on protocol types (TCP, UDP, ICMP), source and destination IP addresses, and specific port numbers. This granularity enables network administrators to restrict access to particular services, such as Telnet, by specifying the TCP destination port 23. Without specifying the port, an ACL entry that denies TCP traffic to a destination IP address will block all TCP-based applications to that host, not just the intended service. When designing ACLs, the decision process involves translating the business or security requirement into precise ACL statements. For example, if the requirement is to block only Telnet access to a server, the ACL must deny TCP traffic specifically to destination port 23 on that server's IP address. Omitting the port qualifier results in a broader deny that matches all TCP traffic, including HTTP, FTP, or other services, which is often unintended. Cisco IOS processes ACL entries sequentially and applies the first matching rule, so specificity and correct ordering are essential to ensure only the targeted traffic is blocked. A common exam trap is failing to include the port number in an extended ACL deny statement, causing the ACL to block more traffic than intended. This mistake stems from misunderstanding how ACL matching works or confusing standard and extended ACL capabilities. Practically, this leads to network disruptions as multiple TCP services become inaccessible. Understanding the difference between standard and extended ACLs, and the importance of port qualifiers in extended ACLs, is critical for precise traffic filtering and passing Cisco CCNA exams confidently.
KKey Concepts to Remember
- An extended ACL in Cisco IOS allows filtering traffic based on protocol types, source and destination IP addresses, and specific TCP or UDP port numbers.
- ACL entries without specific port qualifiers match all traffic of the specified protocol, causing broader traffic blocking than intended.
- The 'host' keyword in ACLs specifies a single IP address and is valid in extended ACLs for precise traffic filtering.
- Telnet uses TCP as its transport protocol, typically on destination port 23, which must be specified in ACLs to restrict only Telnet traffic.
- Standard ACLs filter traffic only by source IP address and cannot filter by protocol or port, making them unsuitable for service-specific restrictions.
- ACLs process entries in sequential order and stop at the first match, so precise ordering and specificity are critical to avoid unintended traffic blocking.
- A broad deny statement in an ACL can inadvertently block multiple TCP-based services, impacting network functionality beyond the intended scope.
- Effective ACL design requires matching the business requirement precisely to the ACL configuration to avoid overblocking or underblocking traffic.
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.
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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 extended ACL in Cisco IOS allows filtering traffic based on protocol types, source and destination IP addresses, and specific TCP or UDP port numbers.
What is the correct answer to this question?
The correct answer is: Because the ACL denies all TCP traffic to the server instead of only Telnet. — The ACL is blocking more traffic than intended because it uses a broad deny against all TCP traffic to the server instead of only the one service that should be denied. In practical terms, the requirement is narrow, but the configured entry is much wider. As a result, multiple TCP-based applications to that server are blocked, not just the intended one. This is a classic precision problem in ACL design. It tests whether you can compare what the business requirement says against what the ACL actually matches.
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.
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