Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
SW1: switchport trunk allowed vlan 10,20,30 SW2: switchport trunk allowed vlan 10,20
Exhibit: Two switches are connected by an 802.1Q trunk. Hosts in VLAN 30 cannot communicate across the link, but VLAN 10 works. 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
Best answer
VLAN 30 is missing from the allowed VLAN list on SW2
SW2 is only allowing VLANs 10 and 20.
B
Distractor review
VLAN 10 must be removed for VLAN 30 to pass
Multiple VLANs can be allowed on a trunk at the same time.
C
Distractor review
The trunk should use ISL instead of 802.1Q
CCNA focuses on 802.1Q and this is not the issue shown.
D
Distractor review
The port on SW1 should be changed to access mode
That would break trunking altogether.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to assume the trunking protocol is the issue and select ISL over 802.1Q. However, CCNA exams expect knowledge of 802.1Q as the standard trunking method, and ISL is deprecated. Another trap is to think that removing VLAN 10 will allow VLAN 30 to pass, but trunks support multiple VLANs simultaneously. Changing the port to access mode is also tempting but incorrect because it disables trunking entirely, blocking all VLANs except the access VLAN. The real cause is usually the allowed VLAN list filtering out VLAN 30 on one switch, which is less obvious but critical to identify.
Technical deep dive
How to think about this question
802.1Q trunking is a fundamental VLAN technology that allows multiple VLANs to share a single physical link between switches by tagging Ethernet frames with VLAN IDs. This tagging enables switches to segregate and forward traffic based on VLAN membership, maintaining logical separation of broadcast domains across the network. Each trunk port has a configuration that includes which VLANs are allowed to pass through it, ensuring only authorized VLAN traffic traverses the link. The allowed VLAN list on a trunk port acts as a filter. Even if a VLAN exists on both switches, if it is not included in the allowed VLAN list on one side, frames tagged for that VLAN will be dropped and not forwarded across the trunk. This is why VLAN 30 traffic fails to communicate across the trunk while VLAN 10 works, indicating VLAN 30 is likely missing from the allowed VLAN list on SW2. The native VLAN is typically untagged and does not affect tagged VLAN traffic, so it is rarely the cause when some VLANs pass and others do not. A common exam trap is to confuse trunking protocols or port modes. For example, suggesting ISL instead of 802.1Q is incorrect because CCNA focuses on 802.1Q as the standard. Similarly, changing a trunk port to access mode disables trunking and prevents multiple VLANs from passing, which is not the solution here. The practical behavior in real networks is to verify allowed VLAN lists on both ends of the trunk to ensure all required VLANs are permitted, which resolves communication issues for specific VLANs like VLAN 30 in this scenario.
KKey Concepts to Remember
- An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
- Switches use an allowed VLAN list on trunk ports to control which VLANs can traverse the trunk; VLANs not in this list are blocked from passing.
- If a VLAN is missing from the allowed VLAN list on one switch, hosts in that VLAN cannot communicate across the trunk despite the physical link being up.
- The native VLAN on an 802.1Q trunk is untagged and usually does not affect VLAN traffic that is tagged and allowed on the trunk.
- Multiple VLANs can coexist on a single 802.1Q trunk link, so removing one VLAN does not enable another VLAN to pass traffic.
- ISL is a Cisco proprietary trunking protocol and is not required or recommended in modern CCNA environments where 802.1Q is standard.
- Access mode ports carry traffic for a single VLAN and do not tag frames; changing a trunk port to access mode disables trunking and blocks multiple VLANs.
- Troubleshooting VLAN communication issues over trunks often involves verifying VLAN membership, trunk configuration, and allowed VLAN lists on both ends.
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 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
What is the correct answer to this question?
The correct answer is: VLAN 30 is missing from the allowed VLAN list on SW2 — If one VLAN crosses the trunk and another does not, the native VLAN is usually not the issue. A far more likely cause is that VLAN 30 is not included in the allowed VLAN list on one or both ends.
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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