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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network technician is configuring an 802.1Q trunk link between core switch `SW-CORE-01` and access switch `SW-ACCESS-02` to allow traffic for multiple VLANs across the site. Interface `GigabitEthernet 1/0/24` on `SW-CORE-01` has been configured with the following command sequence:

text
interface GigabitEthernet 1/0/24
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20,30,40

On `SW-ACCESS-02`, interface `GigabitEthernet 0/1` connects to `SW-CORE-01` and is configured as follows:

text
interface GigabitEthernet 0/1
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20,40

End devices connected to `SW-ACCESS-02` on VLAN 30 are unable to reach the default gateway located on `SW-CORE-01`, while hosts on VLAN 10 and VLAN 20 experience no connectivity issues. Which configuration change on `SW-ACCESS-02` will resolve the connectivity issue for VLAN 30 hosts?

  1. Add VLAN 30 to the allowed VLAN list on interface GigabitEthernet 0/1 of SW-ACCESS-02.Answer
  2. B
    Change the native VLAN on interface GigabitEthernet 0/1 of SW-ACCESS-02 to VLAN 30.
  3. C
    Reconfigure interface GigabitEthernet 0/1 on SW-ACCESS-02 into access mode assigned to VLAN 30.
  4. D
    Modify the native VLAN on SW-CORE-01 to match the default untagged VLAN 1.

Answer

The connectivity issue for VLAN 30 hosts is resolved by adding VLAN 30 to the allowed VLAN list on interface GigabitEthernet 0/1 of SW-ACCESS-02.
The trunk configuration on the access switch explicitly filters out VLAN 30 because its allowed list is limited to VLANs 10, 20, and 40. Updating the allowed VLAN list on interface GigabitEthernet 0/1 of the access switch to include VLAN 30 permits 802.1Q tagged frames for VLAN 30 to pass across the trunk interface to the core switch default gateway.

Step-by-Step Solution

1
Analyze the configuration on both ends of the 802.1Q trunk link.
SW-CORE-01 allows VLANs 10, 20, 30, and 40, while SW-ACCESS-02 only allows VLANs 10, 20, and 40.
When a VLAN is omitted from the trunk's allowed list (`switchport trunk allowed vlan`), the switch drops all ingress and egress frames tagged for that specific VLAN ID on that port.
2
Correlate the symptom with the missing configuration.
Hosts on VLAN 30 attached to SW-ACCESS-02 cannot transmit frames across the trunk to SW-CORE-01 because SW-ACCESS-02 blocks VLAN 30 on interface GigabitEthernet 0/1.
The allowed VLAN list acts as a traffic filter for 802.1Q tagged frames across trunk interfaces.
3
Determine the necessary remediation command.
Modify the trunk configuration on SW-ACCESS-02 to append VLAN 30 to the allowed list (`switchport trunk allowed vlan add 30` or updating the list to include 30).
This allows 802.1Q frames tagged with VLAN 30 to pass bidirectionally across the link.

Key Concept

802.1Q Trunk Allowed VLAN Pruning and Filtering
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Topics

VLAN Configuration and 802.1Q Trunking

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