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Zorluk: OrtaInterswitch Connectivity and Trunking (802.1Q)

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?

  1. VLAN 30 has not been created in the local VLAN database of the switch.Cevap
  2. B
    VLAN 30 is disabled because PortFast is enabled on interface GigabitEthernet0/2.
  3. C
    VLAN 30 is blocked due to a native VLAN ID mismatch on the link.
  4. D
    VLAN 30 was pruned from the link by CDP neighbor discovery messages.

Cevap

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.

Adım Adım Çözüm

1
Analyze the 'Vlans allowed on trunk' output section
VLANs 10, 20, and 30 are configured under the interface `switchport trunk allowed vlan` command.
This section displays the administrative list of allowed VLANs configured on the interface.
2
Analyze the 'Vlans allowed and active in management domain' section
VLAN 30 is absent from this list.
To be active in the management domain, a VLAN must exist in the switch's local VLAN database (`vlan 30`) and not be in a suspended state.
3
Deduce the root cause of the missing VLAN entry
VLAN 30 has not been created using the `vlan 30` global configuration command on the local switch.
Configuring a VLAN in the interface allowed list without creating the VLAN in global configuration prevents the switch from tagging or forwarding frames for that VLAN.

Anahtar Kavram

802.1Q Trunk Allowed vs Active VLAN Operation
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Konular

Interswitch Connectivity and Trunking (802.1Q)

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