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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network administrator is reviewing the switchport configuration on switch `SW-ACCESS-01` after users in the Accounting department (VLAN 20) report a loss of connectivity to servers on the same VLAN across the trunk link to `SW-CORE-01`.

The administrator inspects the output of `show interfaces trunk` on `SW-ACCESS-01` for the inter-switch link interface `GigabitEthernet 0/1`:

text
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1

Port Vlans allowed on trunk
Gi0/1 1-10, 30-100

Which command executed on interface `GigabitEthernet 0/1` of `SW-ACCESS-01` will resolve the connectivity issue for VLAN 20 traffic?

  1. switchport trunk allowed vlan add 20Answer
  2. B
    switchport trunk native vlan 20
  3. C
    switchport mode access
  4. D
    switchport trunk encapsulation isl

Answer

Execute the command `switchport trunk allowed vlan add 20` on interface GigabitEthernet 0/1 to permit VLAN 20 traffic across the trunk link.
The output indicates that VLAN 20 is currently pruned (excluded) from the allowed VLAN list (`1-10, 30-100`) on interface GigabitEthernet 0/1. Issuing the command to add VLAN 20 to the allowed list permits frames tagged with VLAN 20 to be transmitted across the 802.1Q trunk link.

Step-by-Step Solution

1
Analyze the trunk configuration status output
The `Vlans allowed on trunk` list shows `1-10, 30-100`, which explicitly excludes VLAN 20.
When a VLAN is omitted from the allowed list on an 802.1Q trunk, frames tagged for that VLAN are dropped at ingress/egress.
2
Determine the corrective remediation command
Using `switchport trunk allowed vlan add 20` appends VLAN 20 to the existing allowed range without overwriting existing permitted VLANs.
This modifies the interface configuration to permit tagged frames for VLAN 20 across the trunk connection.

Key Concept

802.1Q Trunk Allowed VLAN List Configuration
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Topics

VLAN Configuration and 802.1Q Trunking

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