Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
Requirement: - Block SSH from 10.88.88.0/24 to 192.0.2.10 - Allow all other traffic Configured ACL entry: deny tcp 10.88.88.0 0.0.0.255 host 192.0.2.10
Based on the exhibit, why is this ACL failing to block only SSH traffic from the branch subnet to the router?
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 destination instead of only TCP port 22.
This is correct because the deny statement lacks the SSH destination-port qualifier.
B
Distractor review
Because SSH uses UDP port 22.
This is wrong because SSH uses TCP, not UDP.
C
Distractor review
Because ACLs cannot match host destinations.
This is wrong because host matches are valid and common.
D
Distractor review
Because the ACL should be applied only to IPv6 traffic.
This is wrong because the requirement is clearly an IPv4 extended ACL case.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that a deny statement specifying only the TCP protocol will block just SSH traffic. This mistake ignores that TCP encompasses many services beyond SSH, so the ACL ends up blocking all TCP traffic to the destination. Candidates may overlook the necessity of specifying the destination port 22 to target SSH specifically. This overbroad deny can cause unintended network outages and fails the requirement to block only SSH traffic. Recognizing this subtlety is essential to avoid losing points on ACL precision questions.
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 provide granular control by allowing filtering on Layer 3 and Layer 4 parameters, including TCP or UDP port numbers. This capability is essential when restricting specific services like SSH, which uses TCP port 22, to enforce security policies on routers and switches. When configuring an ACL to block SSH traffic, the rule must explicitly specify TCP as the protocol and port 22 as the destination port. A deny statement that only specifies TCP without the port qualifier will block all TCP traffic to the destination, which is broader than intended. Proper ACL design requires matching the exact service port to avoid unintended traffic disruption. Cisco IOS ACLs process entries sequentially, so specificity in the deny statements ensures only targeted traffic is blocked while other services remain unaffected. The exam trap arises when candidates assume that denying TCP traffic without specifying the port will block only SSH. In reality, this approach blocks all TCP traffic, which can cause network disruptions and does not meet the requirement of blocking only SSH. Practically, this means legitimate TCP services like HTTP or HTTPS could be unintentionally blocked. Understanding the importance of port qualifiers in ACLs is critical for precise traffic filtering and passing Cisco CCNA exams confidently.
KKey Concepts to Remember
- An extended ACL in Cisco IOS can filter traffic based on Layer 3 IP addresses and Layer 4 protocol ports, enabling service-specific traffic control.
- SSH traffic uses TCP as its transport protocol and specifically communicates over TCP port 22, which must be matched in ACL rules to block it accurately.
- A deny ACL statement without a destination port qualifier blocks all TCP traffic to the specified destination, not just SSH traffic.
- Cisco ACLs process entries in sequential order, so placing specific deny statements before broader permits is essential for correct filtering behavior.
- Host destination matches in ACLs are valid and commonly used to precisely identify single devices or interfaces in traffic filtering.
- IPv4 extended ACLs are used to filter IPv4 traffic; applying an ACL designed for IPv4 to IPv6 traffic will not work as expected.
- Failing to specify the correct protocol and port in an ACL can lead to overblocking or underblocking, causing network connectivity issues.
- Precise ACL configuration is critical in security fundamentals to ensure only intended traffic is blocked without disrupting other services.
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 extended ACL in Cisco IOS can filter traffic based on Layer 3 IP addresses and Layer 4 protocol ports, enabling service-specific traffic control.
What is the correct answer to this question?
The correct answer is: Because the ACL denies all TCP traffic to the destination instead of only TCP port 22. — The ACL fails because it blocks all TCP traffic to the router instead of only SSH. In practical terms, the requirement is narrow and service-specific, but the entry is broad. Since SSH is only one TCP-based service, a deny that matches all TCP to the destination is overreaching and blocks more than intended. This is a strong ACL-precision item because it forces you to translate a requirement into the correct degree of specificity.
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.