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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network operations team deploys a new trunk link between access switch SW-Acc-02 and core switch SW-Core-01. Users connected to ports assigned to VLAN 20 on SW-Acc-02 report that they cannot reach servers on VLAN 20 located behind SW-Core-01, while users on VLAN 10 experience normal connectivity.

The current interface configurations are shown below:

SW-Acc-02 Interface Configuration:
text
interface GigabitEthernet1/0/48
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,99

SW-Core-01 Interface Configuration:
text
interface GigabitEthernet0/24
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20,99

Which configuration change on SW-Acc-02 will resolve the loss of connectivity for VLAN 20 traffic across the trunk?

  1. Add VLAN 20 to the switchport trunk allowed vlan list on interface GigabitEthernet1/0/48.Answer
  2. B
    Change the native VLAN on interface GigabitEthernet1/0/48 from VLAN 99 to VLAN 20.
  3. C
    Enable IP routing process on SW-Acc-02 to route VLAN 20 traffic across the trunk.
  4. D
    Reconfigure interface GigabitEthernet1/0/48 as a static access port assigned to VLAN 20.

Answer

Add VLAN 20 to the switchport trunk allowed vlan list on interface GigabitEthernet1/0/48.
The correct answer adds VLAN 20 to the allowed list on SW-Acc-02. An 802.1Q trunk interface only forwards and receives traffic for VLANs specified in its allowed VLAN list. Because SW-Acc-02 was configured to allow only VLANs 10 and 99, all frames for VLAN 20 were dropped at the ingress/egress interface GigabitEthernet1/0/48.

Step-by-Step Solution

1
Analyze the trunk configuration on SW-Acc-02.
Interface GigabitEthernet1/0/48 is configured with 'switchport trunk allowed vlan 10,99', which restricts trunk traffic exclusively to VLAN 10 and VLAN 99.
By default, trunks allow all VLANs unless explicitly filtered using the allowed VLAN list command.
2
Compare SW-Acc-02 allowed VLAN list with SW-Core-01 configuration.
SW-Core-01 allows VLAN 10, 20, and 99, but SW-Acc-02 drops incoming and outgoing VLAN 20 tagged frames at interface GigabitEthernet1/0/48 due to the restrictive allowed list.
802.1Q trunks drop frames tagged with VLAN IDs that are not explicitly included in their allowed VLAN filter.
3
Determine the necessary remediation command.
Executing 'switchport trunk allowed vlan add 20' or modifying the allowed list to '10,20,99' on SW-Acc-02 allows VLAN 20 frames to traverse the trunk link.
Both switches at the ends of an 802.1Q trunk must explicitly permit the VLANs intended to cross the inter-switch link.

Key Concept

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

VLAN Configuration and 802.1Q Trunking

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