Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SW1# 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 SW2# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40,50
Based on the exhibit, what is the strongest explanation for why VLAN 50 hosts on SW1 cannot reach VLAN 50 hosts on SW2?
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 50 is not allowed on the trunk from SW1.
This is correct because VLAN 50 is missing from the allowed list on SW1.
B
Distractor review
The native VLAN must be changed to 50 on both switches.
This is wrong because the problem is the allowed list, not native VLAN selection.
C
Distractor review
The trunk must be changed to an access port.
This is wrong because the link is intended to carry multiple VLANs.
D
Distractor review
The switches must run PPP on the uplink.
This is wrong because PPP is unrelated to Ethernet trunking.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that the native VLAN must be changed to fix VLAN reachability issues. While native VLAN mismatches can cause problems, they do not selectively block a single VLAN’s traffic. Another common mistake is thinking the trunk should be converted to an access port, which disables VLAN tagging and breaks multi-VLAN communication. Candidates may also incorrectly suggest unrelated protocols like PPP, which have no role in Ethernet VLAN trunking. The key trap is overlooking the allowed VLAN list on the trunk, which directly controls VLAN passage and causes selective VLAN failures when misconfigured.
Technical deep dive
How to think about this question
VLAN trunking is a fundamental concept in Cisco switching that allows multiple VLANs to traverse a single physical link between switches. A trunk port carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q. This tagging enables switches to segregate traffic logically while sharing the same physical medium. Proper trunk configuration requires both ends to agree on allowed VLANs, native VLAN, and trunking mode to ensure seamless VLAN communication across switches. The key rule in VLAN trunking is that the trunk port must explicitly allow the VLANs that need to pass through it. Cisco switches use the "switchport trunk allowed vlan" command to specify which VLANs are permitted on a trunk. If a VLAN is omitted from this allowed list on one side of the trunk, frames tagged with that VLAN are dropped and never forwarded across the trunk. This selective filtering causes connectivity issues only for the excluded VLAN, while other VLANs continue to function normally. This behavior is critical for troubleshooting VLAN reachability problems. A common exam trap is to confuse native VLAN mismatches or trunk mode misconfigurations with VLAN filtering issues. While native VLAN mismatches can cause untagged frame drops or security risks, they do not selectively block a single VLAN’s traffic. Similarly, changing a trunk port to an access port disables VLAN tagging entirely, which is not the intended solution when multiple VLANs must traverse the link. Understanding that the allowed VLAN list controls which VLANs pass the trunk is essential to avoid these pitfalls and correctly diagnose VLAN communication failures.
KKey Concepts to Remember
- A trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
- The switchport trunk allowed vlan command controls which VLANs are permitted to traverse a trunk link.
- If a VLAN is missing from the allowed VLAN list on a trunk, frames for that VLAN are dropped and cannot pass between switches.
- Native VLAN mismatches cause untagged frame issues but do not selectively block individual VLAN traffic on a trunk.
- Changing a trunk port to an access port disables VLAN tagging and prevents multiple VLANs from traversing the link.
- Trunk ports must be consistently configured on both ends to ensure VLAN traffic flows correctly across switches.
- Selective VLAN filtering on trunks causes connectivity loss only for the excluded VLAN, while other VLANs remain operational.
- Troubleshooting VLAN reachability requires verifying allowed VLAN lists on trunk ports to identify omitted VLANs.
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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
What is the correct answer to this question?
The correct answer is: VLAN 50 is not allowed on the trunk from SW1. — The strongest explanation is that VLAN 50 is missing from the allowed VLAN list on one side of the trunk. In practical terms, the trunk is up and carrying other VLANs, so the problem is selective rather than total. When one VLAN is omitted from the allowed list, only that VLAN fails while others continue to work normally. This is a high-value switching troubleshooting pattern because it rewards careful reading of operational output rather than generic trunk theory.
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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