Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
access-list 101 permit tcp any any eq 80 interface g0/1 ip access-group 101 in DMZ web server: 172.16.100.10
Exhibit: Users on the inside network can open connections to a web server in the DMZ, but return traffic is denied by an ACL on the outside interface. Which statement best explains the issue?
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
The ACL should match the source port 80 on returning traffic, not the destination port 80
Return packets from the server use source port 80 and a random high destination port on the client.
B
Distractor review
HTTP traffic can never be filtered with a standard ACL
It is true that TCP ports need an extended ACL, but that is not the precise return-traffic explanation here.
C
Distractor review
The ACL must be applied outbound on the inside interface only
ACL placement can vary; the key problem is the match logic.
D
Distractor review
NAT automatically bypasses interface ACLs
NAT does not bypass ACL processing.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is believing that return HTTP traffic will have destination port 80, just like the outbound request. This misconception causes candidates to configure ACLs that only permit inbound packets with destination port 80, which blocks legitimate return traffic because the return packets have source port 80 and a high-numbered destination port. Misunderstanding this port reversal leads to ACLs that deny return traffic, causing connectivity failures despite correct outbound rules. Another trap is confusing NAT behavior, incorrectly assuming NAT bypasses ACLs, which it does not. This misunderstanding can cause candidates to overlook ACL port matching issues.
Technical deep dive
How to think about this question
Access Control Lists (ACLs) are fundamental Cisco security tools used to filter traffic based on IP addresses and TCP/UDP ports. When a client inside a network initiates an HTTP connection to a web server in the DMZ, the outbound packets have a source port assigned by the client (usually a high ephemeral port) and a destination port 80 (HTTP). The return traffic from the server reverses these ports: the source port is 80, and the destination port is the client’s ephemeral port. This port reversal is crucial for ACL matching. Cisco routers apply ACLs based on the direction and interface where they are configured. An inbound ACL on the outside interface inspects packets entering from the DMZ or external network. To permit return HTTP traffic, the ACL must allow packets with source port 80 (from the server) and destination port matching the client’s ephemeral port. If the ACL only permits inbound packets with destination port 80, it will block return traffic because the destination port on return packets is not 80. This is why extended ACLs must be carefully configured to match the correct source and destination ports for bidirectional communication. A common exam trap is assuming that return traffic will have the same destination port as the outbound traffic, leading to ACLs that block legitimate responses. Additionally, NAT does not bypass ACL processing; ACLs still filter packets after NAT translation, so ACL logic must consider translated addresses and ports. Understanding the direction of traffic flow and port roles in TCP connections is essential for correctly configuring ACLs and avoiding connectivity issues in Cisco networks.
KKey Concepts to Remember
- An Access Control List (ACL) evaluates traffic based on source and destination IP addresses and ports to permit or deny packets on Cisco routers.
- Return traffic from a web server uses the server’s source port 80 and a high-numbered ephemeral destination port assigned to the client.
- Extended ACLs must match the correct source and destination ports to allow return traffic through an interface on Cisco devices.
- ACLs applied inbound on an interface filter traffic entering that interface, so return packets must match the ACL’s source port criteria.
- NAT does not bypass ACL processing; ACLs still inspect and filter packets after NAT translation on Cisco routers.
- Incorrect ACL port matching commonly causes return traffic to be denied, even if initial outbound connections are allowed.
- Applying ACLs on the correct interface and direction is critical to ensure bidirectional traffic flow for protocols like HTTP.
- Standard ACLs cannot filter traffic based on TCP or UDP ports; extended ACLs are required for port-specific filtering.
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) evaluates traffic based on source and destination IP addresses and ports to permit or deny packets on Cisco routers.
What is the correct answer to this question?
The correct answer is: The ACL should match the source port 80 on returning traffic, not the destination port 80 — An ACL applied inbound on the outside interface will evaluate the return traffic entering from the DMZ or outside toward the router. If it permits only destination port 80 inbound, the returning packets will not match because their destination is an ephemeral client port, not 80.
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.