An engineer executes the `show interfaces trunk` command on a Cisco Catalyst switch interface GigabitEthernet0/2 and observes the following output:
text
Port Vlans allowed on trunk
Gi0/2 10,20,30
Port Vlans allowed and active in management domain
Gi0/2 10,20
Port Vlans in spanning tree forwarding state and not pruned
Gi0/2 10
Which condition explains why VLAN 30 is listed as allowed on the trunk, but is missing from the list of active VLANs in the management domain?
- VLAN 30 has not been created in the local VLAN database of the switch.Answer
- BVLAN 30 is disabled because PortFast is enabled on interface GigabitEthernet0/2.
- CVLAN 30 is blocked due to a native VLAN ID mismatch on the link.
- DVLAN 30 was pruned from the link by CDP neighbor discovery messages.
Answer
VLAN 30 has not been created in the local VLAN database of the switch.
In Cisco IOS, `show interfaces trunk` divides VLAN status into three operational categories. The first list shows administratively allowed VLANs (`switchport trunk allowed vlan`). The second list shows VLANs that are both administratively allowed AND present in the local switch VLAN database (`vlan <id>`). Because VLAN 30 appears in the first list but not the second, VLAN 30 has not been defined globally in the local switch VLAN database.
Step-by-Step Solution
Key Concept
802.1Q Trunk Allowed vs Active VLAN Operation