Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
SwitchA# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,40 SwitchB# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40
PCs in VLAN 30 on SwitchA cannot reach servers in VLAN 30 on SwitchB. All other VLANs work across the trunk. What is the most likely cause?
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 native VLAN is mismatched between the switches.
This is wrong because both outputs show native VLAN 1. A native VLAN mismatch is not shown here.
B
Distractor review
VLAN 30 should be configured as the native VLAN on both ends.
This is wrong because VLAN 30 does not need to become the native VLAN in order to traverse the trunk.
C
Distractor review
SwitchB must use ISL instead of 802.1Q.
This is wrong because 802.1Q is already working for the other VLANs. The issue is not encapsulation type.
D
Best answer
VLAN 30 is not allowed on the trunk from SwitchA.
This is correct because the exhibit clearly shows VLAN 30 missing from the allowed list on SwitchA. A VLAN must be permitted across the trunk on both sides for end-to-end communication.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to assume that a native VLAN mismatch or trunk encapsulation type is causing VLAN connectivity issues. Candidates might incorrectly select options related to native VLAN changes or switching from 802.1Q to ISL because these concepts are often tested. However, if other VLANs are passing through the trunk successfully, it indicates that the trunk is operational and the native VLAN is consistent. The real issue lies in the allowed VLAN list, which can silently block specific VLANs like VLAN 30 if not configured on both trunk endpoints. Overlooking this detail leads to incorrect troubleshooting and answer selection.
Technical deep dive
How to think about this question
VLAN trunking is a fundamental concept in Cisco networking that allows multiple VLANs to be carried over a single physical link between switches. The most common trunking protocol used in CCNA environments is IEEE 802.1Q, which tags Ethernet frames with VLAN identifiers to maintain VLAN separation across the trunk. For end-to-end VLAN communication, both switches must agree on which VLANs are allowed to traverse the trunk link. If a VLAN is not permitted on the trunk, frames tagged with that VLAN ID will be dropped, preventing communication between devices in that VLAN across switches. When configuring trunks, Cisco switches use the "allowed VLAN" list to control which VLANs can pass through the trunk port. This list must be consistent on both ends of the trunk link to ensure VLAN traffic flows correctly. A mismatch where one switch allows a VLAN and the other does not will cause connectivity issues for that VLAN. The native VLAN setting, which is untagged on 802.1Q trunks, must also match but does not affect VLANs that are explicitly tagged. Therefore, missing VLAN 30 from the allowed VLAN list on SwitchA blocks VLAN 30 traffic, even though other VLANs work fine. A common exam trap is to focus on native VLAN mismatches or encapsulation types like ISL versus 802.1Q, but these are not the cause when other VLANs traverse the trunk successfully. The practical impact is that VLAN 30 traffic is effectively filtered out by SwitchA’s trunk configuration, causing devices in VLAN 30 on SwitchA to be unable to reach VLAN 30 devices on SwitchB. Correcting the allowed VLAN list to include VLAN 30 on both switches restores proper VLAN communication across the trunk.
KKey Concepts to Remember
- A VLAN trunk carries multiple VLANs over a single physical link by tagging frames with VLAN identifiers using 802.1Q encapsulation.
- Both ends of a trunk link must have matching allowed VLAN lists to permit VLAN traffic to pass between switches.
- If a VLAN is missing from the allowed VLAN list on one switch, traffic for that VLAN is blocked on the trunk, preventing inter-switch communication.
- The native VLAN is the untagged VLAN on an 802.1Q trunk and must match on both ends, but it does not affect tagged VLAN traffic.
- 802.1Q is the standard trunking protocol in Cisco switches and does not require switching to ISL unless specifically needed.
- A native VLAN mismatch typically causes broader connectivity issues and is unlikely if other VLANs are passing through the trunk successfully.
- The show interfaces trunk command displays the allowed VLANs and native VLAN, aiding in troubleshooting VLAN trunk issues.
- Consistent trunk configuration on both switches is essential to maintain VLAN segmentation and ensure proper inter-VLAN communication.
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?
A VLAN trunk carries multiple VLANs over a single physical link by tagging frames with VLAN identifiers using 802.1Q encapsulation.
What is the correct answer to this question?
The correct answer is: VLAN 30 is not allowed on the trunk from SwitchA. — The trunk is up and carrying multiple VLANs, but VLAN 30 is missing from the allowed list on SwitchA. In simple terms, both switches have opened the hallway between them, but one side is refusing to let VLAN 30 traffic through that hallway. Since SwitchB allows VLAN 30 and SwitchA does not, traffic for that VLAN cannot cross the trunk successfully from end to end. The output also shows that the native VLAN is 1 on both switches, so there is no native VLAN mismatch. Cisco switches commonly use 802.1Q for trunking, so there is no requirement to switch to ISL here. The issue is much narrower: the allowed VLAN list is inconsistent. Adding VLAN 30 to the trunk allowed list on SwitchA would align the configuration and restore connectivity for that VLAN.
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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