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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An administrator executes the command `show interfaces trunk` on a Cisco Catalyst switch and notices that VLAN 50 is included in the "VLANs allowed on trunk" section, but it does not appear in the "VLANs allowed and active in management domain" section. Which condition accounts for this operational state?

  1. VLAN 50 has not been created in the switch's local VLAN database.Answer
  2. B
    A native VLAN mismatch on the trunk link prevents VLAN 50 from being activated.
  3. C
    Spanning Tree PortFast is enabled on the interface, which automatically prunes VLAN 50.
  4. D
    CDP and LLDP negotiation failed to synchronize the active VLAN list across the link.

Answer

VLAN 50 has not been created in the switch's local VLAN database.
For a VLAN to appear under 'VLANs allowed and active in management domain' in the `show interfaces trunk` output, it must be permitted by the trunk allowed list AND exist locally in the switch's VLAN database. If the VLAN has not been created using the `vlan 50` global configuration command, it remains inactive on the switch and will not appear in the active management domain output.

Step-by-Step Solution

1
Analyze the operational sections of the `show interfaces trunk` CLI output.
The 'VLANs allowed on trunk' output reflects the interface configuration (e.g., `switchport trunk allowed vlan`), while 'VLANs allowed and active in management domain' shows VLANs from that allowed list that currently exist in the local VLAN database and are not in a shut-down state.
Understanding Cisco IOS trunk verification logic is essential for accurately interpreting command output.
2
Identify why an allowed VLAN is excluded from the active management domain.
Because VLAN 50 is allowed by configuration but absent from the active list, it has not been instantiated in the switch database (using `vlan 50` in global configuration mode) or is currently disabled.
A switch cannot forward or locally activate trunk traffic for a VLAN that does not exist in its VLAN database.

Key Concept

802.1Q Trunk Verification and Local VLAN Database Dependencies
Estimated Time:1m 15s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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