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Zorluk: OrtaInterswitch Connectivity and Trunking (802.1Q)

An administrator is configuring a trunk link between Switch-1 and Switch-2. On Switch-1 interface GigabitEthernet0/1, the engineer executes the following configuration commands:

Switch-1(config-if)# switchport mode trunk
Switch-1(config-if)# switchport trunk native vlan 20
Switch-1(config-if)# switchport trunk allowed vlan 10,30,40

Assuming Switch-2 has default 802.1Q trunk configurations with native VLAN 20 allowed, how will Switch-1 process frame forwarding for VLAN 20 traffic egressing interface GigabitEthernet0/1?

  1. Switch-1 drops frames originating from VLAN 20 because VLAN 20 is not included in the interface allowed VLAN list.Cevap
  2. B
    Switch-1 forwards VLAN 20 frames untagged because VLAN 20 is configured as the native VLAN on the interface.
  3. C
    Switch-1 tags VLAN 20 frames with an 802.1Q header before transmitting them across the link.
  4. D
    Switch-1 automatically enables PortFast on GigabitEthernet0/1 to permit untagged native VLAN traffic.

Cevap

Switch-1 drops frames originating from VLAN 20 because VLAN 20 is explicitly excluded from the trunk's allowed VLAN list.
The correct answer identifies that Switch-1 drops frames originating from VLAN 20. When the command 'switchport trunk allowed vlan' is used with an explicit parameter list, only traffic for the listed VLANs (10, 30, 40) is permitted across the trunk link. Because VLAN 20 is not in this list, its traffic is pruned and dropped at ingress/egress, even though it is configured as the native VLAN.

Adım Adım Çözüm

1
Analyze the configuration command 'switchport trunk allowed vlan 10,30,40'.
The allowed VLAN list for interface GigabitEthernet0/1 is set exclusively to VLANs 10, 30, and 40.
Explicitly specifying allowed VLAN numbers replaces the default 'all' list with only the specified VLAN IDs.
2
Evaluate the status of native VLAN 20 against the allowed VLAN list.
VLAN 20 is configured as native, but it is absent from the allowed list (10, 30, 40).
For a native VLAN's traffic to transit an 802.1Q trunk link, it must be included in the port's allowed VLAN list.
3
Determine the forwarding behavior for egress VLAN 20 frames on Switch-1.
Switch-1 drops egress frames belonging to VLAN 20.
Trunk interfaces filter out (prune) any frames belonging to VLANs not present in the allowed list, regardless of native VLAN designation.

Anahtar Kavram

802.1Q Trunk Allowed VLAN Filtering Logic
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Konular

Interswitch Connectivity and Trunking (802.1Q)

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