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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network technician is troubleshooting connectivity issues following a switch maintenance window. Workstations assigned to VLAN 45 on an edge switch can no longer communicate with the default gateway hosted on the core switch, whereas hosts on VLAN 10 attached to the same edge switch maintain uninterrupted access. The trunk link between the switches is configured using 802.1Q. Output from `show interfaces trunk` on the core switch shows the allowed VLAN list on interface GigabitEthernet0/1 is set to `10,20,30`. Which of the following configuration changes on the core switch will resolve the connectivity issue for VLAN 45?

  1. Add VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1.Answer
  2. B
    Reconfigure the native VLAN on interface GigabitEthernet0/1 to VLAN 45.
  3. C
    Configure an inter-VLAN routing interface directly on the access switch for VLAN 45.
  4. D
    Set the Spanning Tree Protocol priority for VLAN 45 on the access switch to 0.

Answer

Add VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1.
On 802.1Q trunk links, the allowed VLAN list specifies which VLAN tags the switch interface will accept and forward. Since `show interfaces trunk` shows the allowed list restricted to VLANs 10, 20, and 30, traffic tagged for VLAN 45 is dropped at the switch port. Adding VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1 permits frames tagged for VLAN 45 to traverse the trunk link to reach the default gateway.

Step-by-Step Solution

1
Analyze the symptom and output
VLAN 10 traffic succeeds while VLAN 45 traffic fails across the trunk link. Interface `GigabitEthernet0/1` trunk status explicitly lists allowed VLANs as `10,20,30`.
By default, 802.1Q trunks allow all VLANs (1-4094), but explicit `switchport trunk allowed vlan` lists prune unlisted VLANs from entering or exiting the interface.
2
Identify the missing configuration element
VLAN 45 is absent from the allowed list, causing the switch interface to drop all incoming and outgoing frames carrying the 802.1Q tag for VLAN 45.
Traffic for VLAN 45 cannot reach the core switch's default gateway because the trunk port prunes the traffic at Layer 2.
3
Select the corrective CLI configuration
Executing `switchport trunk allowed vlan add 45` on interface GigabitEthernet0/1 includes VLAN 45 in the trunk's active forwarding list.
Modifying the allowed list restores Layer 2 trunking for tagged VLAN 45 frames without disrupting existing allowed VLANs.

Key Concept

VLAN Trunking Allowed List Configuration and Pruning
Estimated Time:1m 30s
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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