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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

An engineer is troubleshooting a newly configured IEEE 802.1Q trunk link on interface `GigabitEthernet 0/2` connecting two switches, `SW-DIST-01` and `SW-ACCESS-01`. Users connected to VLAN 50 report a total loss of inter-switch connectivity, whereas users on VLAN 10 and VLAN 20 experience normal operations across the trunk. The interface configuration on `SW-DIST-01` is shown below:

text
interface GigabitEthernet0/2
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20,99

Which configuration command should the engineer execute on `SW-DIST-01` to restore inter-switch connectivity for VLAN 50?

  1. switchport trunk allowed vlan add 50Answer
  2. B
    switchport trunk native vlan 50
  3. C
    switchport mode access
  4. D
    switchport trunk allowed vlan 50

Answer

Execute `switchport trunk allowed vlan add 50` on interface `GigabitEthernet 0/2` to permit VLAN 50 traffic while preserving existing trunked VLANs.
The command `switchport trunk allowed vlan add 50` appends VLAN 50 to the active trunk filter list. Because the current interface explicitly restricts allowed VLANs to 10, 20, and 99, VLAN 50 traffic is dropped until it is explicitly appended to the permitted list.

Step-by-Step Solution

1
Analyze the existing running configuration snippet
The `switchport trunk allowed vlan 10,20,99` line restricts frame transmission across the trunk to only VLANs 10, 20, and 99.
By default, an 802.1Q trunk allows all VLANs (1-4094) unless an allowed VLAN list filter is configured.
2
Identify the cause of traffic dropping for VLAN 50
Frames tagged with VLAN ID 50 arriving at or departing from interface `GigabitEthernet 0/2` are filtered and discarded due to missing authorization in the allowed list.
VLAN 50 is omitted from the current allowed VLAN list.
3
Determine the appropriate CLI syntax modification
Using `switchport trunk allowed vlan add 50` appends VLAN 50 to the existing list (10,20,50,99).
Omitting the `add` keyword would overwrite the entire allowed list and cause outages for VLANs 10 and 20.

Key Concept

802.1Q Trunk Allowed VLAN Filtering
Estimated Time:1m 30s
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Topics

VLAN Configuration and 802.1Q Trunking

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