Trunk PVID Mismatch and VLAN 10 Forwarding on LSW1/LSW2
Short answer
B The statement is FALSE because LSW1's G0/0/1 trunk PVID VLAN 20 and LSW2's PVID VLAN 10 cause VLAN 10 frames from LSW2 to egress untagged and be dropped after LSW1 assigns them to VLAN 20.
Question
Key Takeaways
It tests whether you recognize that a trunk's PVID controls whether a VLAN tag is stripped on egress, so mismatched PVIDs can silently misclassify VLAN 10 frames even when VLAN 10 is allowed.
This H12-811 item tests trunk PVID behavior on Huawei switches; the answer is FALSE (B) because the mismatched native VLANs prevent VLAN 10 traffic from crossing the link correctly.
Common mistake: A common wrong answer is TRUE, assuming that `allow-pass vlan 10` alone guarantees VLAN 10 forwarding; it ignores that LSW2's PVID VLAN 10 strips the tag and LSW1's PVID VLAN 20 reassigns it.
Expert Analysis
The governing rule: PVID controls tag handling on a trunk.
The deciding principle is not the allow-pass vlan 10 command alone; it is how a Huawei trunk treats its PVID, also called the native VLAN. When a frame leaves a trunk port, the switch removes the VLAN tag if the frame belongs to the port's PVID and sends other allowed VLANs tagged. In the reverse direction, an untagged frame arriving on the trunk is placed into the local PVID, while a tagged frame is accepted only if its VLAN is allowed on that trunk.
Mapped to LSW1 and LSW2. LSW1's G0/0/1 is configured with PVID VLAN 20 and allows VLAN 10. LSW2's G0/0/1 is configured with PVID VLAN 10 and allows VLAN 10. A VLAN 10 frame leaving LSW2 is therefore sent untagged because VLAN 10 is LSW2's PVID. When LSW1 receives that untagged frame, it assigns it to VLAN 20, its own PVID, and VLAN 20 is not permitted on the VLAN 10-only trunk. The frame cannot be forwarded normally. In the other direction, a VLAN 10 frame from LSW1 would remain tagged because LSW1's PVID is 20, but the return path still fails, so end-to-end forwarding is not normal.
Verdict and trap. The statement says VLAN 10 tagged frames can be normally forwarded between LSW1 and LSW2. That is false, so option B is correct and option A is the trap. The supplied discussion makes the same point: the two trunk PVIDs differ, and the VLAN 10 frame is untagged by LSW2 and reclassified into VLAN 20 at LSW1. A matching PVID is needed when the native VLAN is expected to carry the same traffic; otherwise the allowed VLAN list alone does not guarantee correct forwarding.
Exam Strategy
On trunk questions, separate the three decisions: link type, allowed VLAN list, and PVID. Check the PVID on both ends before accepting a claim that a tagged VLAN can pass, because the egress side may strip the tag and the ingress side may assign a different VLAN. Trace the frame path direction by direction and verify the final VLAN is in the allow-pass list.
Frequently Asked Questions
Why does LSW2 send VLAN 10 frames untagged on G0/0/1?
If both trunks allow VLAN 10, why is the traffic still dropped?
Does `allow-pass vlan 10` guarantee that VLAN 10 can cross a trunk?
Community Votes
100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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4243643 👍 1 Answered BThe statement is FALSE because the trunk links have different PVIDs. On LSW1, the PVID is VLAN 20. On LSW2, the PVID is VLAN 10. Both trunks allow only VLAN 10. When a VLAN 10 frame is sent from LSW2, it is transmitted untagged because VLAN 10 is the PVID. LSW1 receives the untagged frame and assigns it to its own PVID, VLAN 20. Since VLAN 20 is not allowed on the trunk, the traffic cannot be forwarded correctly. Therefore, VLAN 10 traffic cannot be normally forwarded between LSW1 and LSW2, so the correct answer is B. FALSE.
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