Question 219 of 1,389
CCNA Switching and Network Access Practice Question
After configuring a trunk port to allow VLAN 40, a technician finds that VLAN 40 is not listed among the VLANs in spanning tree forwarding state in the show interfaces trunk output. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between allowing a VLAN on a trunk and actually creating the VLAN in the VLAN database—candidates mistakenly assume that configuring the trunk alone is sufficient for the VLAN to be operational.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 40 has not been created in the VLAN database on the switch.
VLAN 40 must exist in the local VLAN database before it can participate in spanning tree on a trunk port. Even if the trunk is configured to allow VLAN 40, if the VLAN has not been created on the switch, the spanning tree protocol will not place it in a forwarding state. The 'show interfaces trunk' output will list only VLANs that are both allowed and existent, so the absence of VLAN 40 in the forwarding state indicates it was never created.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The trunk port is using ISL encapsulation, which does not support VLAN 40.
Why it's wrong here
ISL encapsulation supports VLANs 1–1005, so VLAN 40 is within the valid range. Modern Cisco switches default to 802.1Q, and if encapsulation were mismatched the trunk would likely be down. The root cause is not encapsulation.
- ✗
The technician omitted the 'add' keyword when adding VLAN 40 to the allowed list, so the trunk no longer permits VLAN 40.
Why it's wrong here
When the 'add' keyword is omitted, the allowed list is overwritten and all previously allowed VLANs are removed. In that case, the show interfaces trunk command would not show VLAN 40 in the allowed list at all, contradicting the symptom of it being allowed but not in forwarding state.
- ✓
VLAN 40 has not been created in the VLAN database on the switch.
Why this is correct
A VLAN must be defined in the local VLAN database for the switch to build a spanning-tree instance and forward frames for that VLAN. If it is permitted on the trunk but does not exist, the switch marks it as pruned and it will not appear in the 'VLANs in spanning tree forwarding state' list. This is the exact symptom presented.
- ✗
VTP pruning is enabled, and VLAN 40 is not needed by any downstream neighbor, so it is pruned from this trunk.
Why it's wrong here
VTP pruning removes VLAN traffic from trunks when no downstream switch has ports in that VLAN, but it requires a VTP domain and is a separate feature. Without any indication of VTP configuration, the most likely reason a single VLAN is missing from the forwarding list after being allowed is that the VLAN itself has not been created.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓VLAN 40 has not been created in the VLAN database on the switch.Correct answer▾
Why this is correct
A VLAN must be defined in the local VLAN database for the switch to build a spanning-tree instance and forward frames for that VLAN. If it is permitted on the trunk but does not exist, the switch marks it as pruned and it will not appear in the 'VLANs in spanning tree forwarding state' list. This is the exact symptom presented.
✗The trunk port is using ISL encapsulation, which does not support VLAN 40.Wrong answer — click to see why▾
Why this is wrong here
Candidates might associate VLAN support with trunk encapsulation types, but ISL fully supports VLAN 40. This is a distractor.
✗The technician omitted the 'add' keyword when adding VLAN 40 to the allowed list, so the trunk no longer permits VLAN 40.Wrong answer — click to see why▾
Why this is wrong here
This is a common operational mistake, but the resulting output would show VLAN 40 missing from the 'Vlans allowed' column, not from the forwarding list.
✗VTP pruning is enabled, and VLAN 40 is not needed by any downstream neighbor, so it is pruned from this trunk.Wrong answer — click to see why▾
Why this is wrong here
Candidates might confuse local pruning (due to non-existent VLAN) with VTP pruning. VTP pruning would also require a multi-switch VTP domain and is less likely in a standalone troubleshooting scenario.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Last reviewed: Jul 4, 2026
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