- A
The trunk port is using ISL encapsulation, which does not support VLAN 40.
Why wrong: 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.
- B
The technician omitted the 'add' keyword when adding VLAN 40 to the allowed list, so the trunk no longer permits VLAN 40.
Why wrong: 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.
- C
VLAN 40 has not been created in the VLAN database on the switch.
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.
- D
VTP pruning is enabled, and VLAN 40 is not needed by any downstream neighbor, so it is pruned from this trunk.
Why wrong: 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.
Quick Answer
The answer is that VLAN 40 has not been created in the VLAN database on the switch. Even if you configure a trunk port to allow a specific VLAN with the switchport trunk allowed vlan add command, the switch cannot forward frames for that VLAN unless the VLAN exists in the local VLAN database. A non-existent VLAN is automatically placed in a pruned state, so it will not appear in the spanning-tree forwarding state within the show interfaces trunk output—the allowed-list command worked, but the missing VLAN definition prevents it from becoming active on the trunk. On the CCNA 200-301 v2 exam, this scenario tests your understanding that trunk configuration and VLAN creation are separate steps; a common trap is assuming that adding a VLAN to the allowed list also creates it. Remember the memory tip: “Allow it, but create it first—no database, no state.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Even if a VLAN is included in the trunk's allowed list, the switch cannot forward frames for that VLAN unless it exists in the local VLAN database. A non-existent VLAN is placed in a pruned state and will not appear as forwarding in show interfaces trunk. The allowed-list command worked, but the missing VLAN definition prevents the VLAN from being active on the trunk.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
- ✗
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
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?”
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Keyword trap
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.
Command / output trap
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.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: VLAN 40 has not been created in the VLAN database on the switch. — Even if a VLAN is included in the trunk's allowed list, the switch cannot forward frames for that VLAN unless it exists in the local VLAN database. A non-existent VLAN is placed in a pruned state and will not appear as forwarding in show interfaces trunk. The allowed-list command worked, but the missing VLAN definition prevents the VLAN from being active on the trunk.
What should I do if I get this 200-301 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 14, 2026
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