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Zorluk: OrtaVLAN Configuration and 802.1Q Trunking

A network engineer is configuring switch interface `GigabitEthernet1/0/12` on `SW-DIST-05` to connect a multi-SSID wireless access point. Management traffic from the access point must travel untagged on VLAN 15, while client traffic is tagged on VLAN 45 (Corporate) and VLAN 85 (Guest).

Initially, the interface was configured as follows:
text switchport mode trunk switchport trunk native vlan 15 switchport trunk allowed vlan 15,45,85
During a maintenance window, a junior technician updates the interface configuration by executing:
text switchport trunk allowed vlan 45,85
Which of the following describes the immediate operational outcome of this configuration change?

  1. Untagged management traffic on VLAN 15 will be dropped at the switchport because VLAN 15 is missing from the allowed VLAN list.Cevap
  2. B
    Untagged management traffic on VLAN 15 will be automatically re-tagged and forwarded as VLAN 45 upon entering the switch interface.
  3. C
    Tagged client traffic on VLAN 45 and VLAN 85 will be blocked because the native VLAN must be present in the allowed VLAN list for trunking to operate.
  4. D
    Devices on VLAN 45 and VLAN 85 will begin communicating directly across the trunk without requiring Layer 3 routing.

Cevap

Untagged management traffic on VLAN 15 will be dropped at the switchport because VLAN 15 is missing from the allowed VLAN list.
On an 802.1Q trunk, the interface filters both tagged and untagged traffic using the allowed VLAN list. When the native VLAN (VLAN 15) is removed from the allowed VLAN list, untagged management frames arriving from the wireless access point are dropped by the switch interface because VLAN 15 is no longer permitted on the link.

Adım Adım Çözüm

1
Analyze the original trunk configuration.
Interface `GigabitEthernet1/0/12` was configured to use VLAN 15 as the native VLAN, allowing untagged frames from the AP to be processed in VLAN 15. The allowed list explicitly permitted VLANs 15, 45, and 85.
By default, an 802.1Q trunk processes untagged ingress frames into the designated native VLAN.
2
Evaluate the impact of the modification command.
The technician ran `switchport trunk allowed vlan 45,85`, overwriting the allowed list and removing VLAN 15.
On Cisco switches, `switchport trunk allowed vlan <list>` replaces the current allowed VLAN set rather than appending to it unless the `add` keyword is used.
3
Determine the forwarding behavior for native VLAN traffic when excluded from the allowed list.
Because VLAN 15 is no longer in the allowed list, any untagged ingress or egress frames associated with native VLAN 15 are pruned/dropped at the trunk port.
An 802.1Q trunk interface only forwards traffic for VLANs present in its active allowed VLAN list, regardless of the native VLAN setting.

Anahtar Kavram

802.1Q Trunk Allowed VLAN Pruning and Native VLAN Processing
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Konular

VLAN Configuration and 802.1Q Trunking

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