A network technician receives a ticket stating that IP phones on VLAN 30 attached to Switch-B cannot communicate with the VoIP call manager connected to Switch-A. However, workstations on VLAN 10 attached to the same switches are communicating across the interconnecting 802.1Q trunk link without any issues.
An inspection of Switch-B's trunk port configuration reveals the following entry:
`switchport trunk allowed vlan 10,20`
Which of the following identifies the root cause of the connectivity failure for VLAN 30 traffic?
- VLAN 30 is absent from the trunk interface allowed list on Switch-B, causing the switch to drop all frames tagged for VLAN 30 at the trunk border.Answer
- BSwitch-B has assigned VLAN 30 as its native VLAN, which strips 802.1Q tags from VoIP packets and forwards them onto VLAN 10.
- CSpanning Tree Protocol placed the trunk port into a blocking state because Switch-B failed to win the STP Root Bridge election for VLAN 30.
- DA speed and duplex mismatch on the trunk port is selectively dropping high-priority Voice over IP packets while allowing standard data frames to pass.
Answer
VLAN 30 is absent from the trunk interface allowed list on Switch-B, causing the switch to drop all frames tagged for VLAN 30 at the trunk border.
The correct answer identifies that VLAN 30 is excluded from the allowed VLAN list on Switch-B's trunk port. When `switchport trunk allowed vlan` is executed, it overrides the default behavior (which allows all VLANs) and strictly restricts trunk traffic to only the specified VLANs (in this case, 10 and 20). As a result, all VLAN 30 frames are filtered and dropped at the trunk interface.
Step-by-Step Solution
Key Concept
VLAN Trunk Allowed List Filtering