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Zorluk: OrtaInterswitch Connectivity and Trunking (802.1Q)

An engineer executes the command `show interfaces gigabitethernet 0/1 switchport` on a Cisco Catalyst switch and receives the following output:

text
Name: Gi0/1
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (VLAN0010)
Administrative Native VLAN tagging: enabled
Trunking VLANs Enabled: 10, 20, 30

How will the switch handle an outgoing frame belonging to VLAN 10 as it egresses interface GigabitEthernet0/1?

  1. The frame will be transmitted with an explicit 802.1Q VLAN tag containing VLAN ID 10 because global native VLAN tagging is enabled.Cevap
  2. B
    The frame will be transmitted untagged because VLAN 10 is designated as the trunk interface native VLAN.
  3. C
    The frame will be dropped at the interface because PortFast is enabled on trunk interfaces by default.
  4. D
    The frame will be encapsulated with a 26-byte ISL header because dynamic encapsulation negotiation is active.

Cevap

The switch will transmit the frame with an explicit 802.1Q tag for VLAN 10 because global native VLAN tagging is active on the switch.
Standard IEEE 802.1Q trunking sends frames belonging to the native VLAN without a VLAN tag. However, when native VLAN tagging is enabled globally (`vlan dot1q tag native`), the switch appends an 802.1Q tag header (VLAN ID 10) to all outgoing native VLAN frames across 802.1Q trunks.

Adım Adım Çözüm

1
Analyze the operational parameters in the CLI output.
The interface is operating as an 802.1Q trunk ('Operational Mode: trunk', 'Operational Trunking Encapsulation: dot1q') with VLAN 10 configured as the native VLAN ('Trunking Native Mode VLAN: 10').
Understanding the interface trunk encapsulation and native VLAN assignment sets the baseline for frame processing.
2
Evaluate the native VLAN tagging state.
The output states 'Administrative Native VLAN tagging: enabled', indicating that the global command 'vlan dot1q tag native' is in effect.
By default, 802.1Q native VLAN frames are transmitted untagged, but enabling global native VLAN tagging forces all native VLAN egress frames to be explicitly tagged.

Anahtar Kavram

802.1Q Native VLAN Tagging Behavior
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Konular

Interswitch Connectivity and Trunking (802.1Q)

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