An enterprise network engineer is provisioning a switch interface connecting switch SW-Floor3 to the distribution switch SW-Dist1. The link carries traffic for VLAN 10 (Corporate Data), VLAN 20 (Guest Wi-Fi), and VLAN 50 (VoIP Telephony). IP phones connected to SW-Floor3 fail to register with the central call manager on VLAN 50, although workstations on VLAN 10 communicate across the link normally.
The engineer views the trunk configuration on interface GigabitEthernet1/0/48:
text
interface GigabitEthernet1/0/48
switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk native vlan 10
switchport trunk allowed vlan 10,20,30
Which of the following identifies the root cause of the VoIP communication failure?
- The allowed VLAN pruning list on the trunk interface omits VLAN 50, preventing traffic belonging to the voice network from traversing the link.Answer
- BConfiguring VLAN 10 as the native VLAN causes all VLAN 50 tagged frames to be automatically re-tagged as VLAN 10 before transmission.
- CVLAN 10 and VLAN 50 belong to different broadcast domains and cannot both traverse an 802.1Q trunk link without a Layer 3 router on the interface.
- DThe 802.1Q encapsulation command disables voice VLAN tagging, converting all voice frames into untagged traffic that gets dropped by the remote switch.
Answer
The allowed VLAN pruning list on the trunk interface omits VLAN 50, preventing traffic belonging to the voice network from traversing the link.
The configuration command 'switchport trunk allowed vlan 10,20,30' creates an explicit permit list for trunking. Because VLAN 50 is not included in this allowed list, the switch prunes VLAN 50 traffic, dropping all ingress and egress 802.1Q tagged frames associated with the voice network on this port.
Step-by-Step Solution
Key Concept
802.1Q Trunk Allowed VLAN Filtering