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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network technician is troubleshooting an inter-switch connectivity issue. Host PC-A (IP address 192.168.10.15/24) connected to switch SW-EDGE-01 in VLAN 10 cannot communicate with Host PC-B (IP address 192.168.10.25/24) connected to switch SW-EDGE-02 in VLAN 10. The two switches are interconnected via an 802.1Q trunk link on interface GigabitEthernet0/24.

Upon inspecting interface GigabitEthernet0/24 on SW-EDGE-01, the technician reviews the following command output:

SW-EDGE-01# show interfaces gigabitethernet0/24 switchport
Name: Gi0/24
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: disabled
Voice VLAN: none
Administrative Mode VLANs Allowed: 1-9,11-4094

Which of the following configuration commands on interface GigabitEthernet0/24 of SW-EDGE-01 will resolve the communication failure between PC-A and PC-B?

  1. `switchport trunk allowed vlan add 10`Answer
  2. B
    `switchport trunk native vlan 10`
  3. C
    `switchport mode access`
  4. D
    `switchport access vlan 10`

Answer

Executing `switchport trunk allowed vlan add 10` on interface GigabitEthernet0/24 appends VLAN 10 to the allowed list, enabling VLAN 10 traffic to traverse the trunk link.
The output demonstrates that interface GigabitEthernet0/24 has an administrative allowed VLAN list of `1-9,11-4094`. Because VLAN 10 is excluded from this list, switch SW-EDGE-01 prunes all ingress and egress frames belonging to VLAN 10 on this trunk interface. Executing the command `switchport trunk allowed vlan add 10` explicitly appends VLAN 10 to the permitted range, allowing traffic between PC-A and PC-B to pass across the switch link.

Step-by-Step Solution

1
Analyze the output of `show interfaces gigabitethernet0/24 switchport`.
The parameter `Administrative Mode VLANs Allowed` is set to `1-9,11-4094`.
VLAN 10 is omitted from the allowed range, causing the switch to drop all tagged frames for VLAN 10 arriving at or departing from interface GigabitEthernet0/24.
2
Determine the corrective Cisco IOS switchport configuration command.
Using `switchport trunk allowed vlan add 10` modifies the trunk filter to permit VLAN 10 frames.
Using the `add` keyword ensures that VLAN 10 is appended to the current list of allowed VLANs without replacing the existing list.

Key Concept

802.1Q Trunk Allowed VLAN List Pruning and Configuration
Estimated Time:1m 15s
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Topics

VLAN Configuration and 802.1Q Trunking

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