Examkin

Question

Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

A network administrator executes the global configuration command `vlan dot1q tag native` on a Cisco Catalyst switch. Interface GigabitEthernet1/0/1 on the switch is explicitly configured with `switchport mode trunk` and `switchport trunk native vlan 50`. Which TWO operational behaviors occur on this trunk interface as a result of this configuration? (Select TWO.)

  1. Egress frames originating from VLAN 50 exiting interface GigabitEthernet1/0/1 are encapsulated with an 802.1Q header specifying VLAN ID 50.Answer
  2. Ingress untagged Ethernet frames received on interface GigabitEthernet1/0/1 are dropped by the switch interface.Answer
  3. C
    Frames belonging to VLAN 50 continue to be transmitted untagged while all other allowed VLANs are dropped until CDP renegotiates the native VLAN ID.
  4. D
    Interface GigabitEthernet1/0/1 is automatically forced into Spanning Tree PortFast mode to prevent native VLAN convergence delays.
  5. E
    CDP Type-Length-Value (TLV) messages dynamically alter the neighboring switch running configuration to match the native VLAN tagging state.

Answer

Enabling the global command `vlan dot1q tag native` forces the switch to place an 802.1Q tag on all native VLAN traffic (VLAN 50) egressing trunk ports and causes the switch to drop incoming untagged frames received on those trunk ports.
Under default 802.1Q operation, native VLAN traffic is sent untagged. Executing `vlan dot1q tag native` globally forces the switch to insert an 802.1Q tag into egress frames for the native VLAN (VLAN 50) and drop any untagged frames received on trunk interfaces.

Step-by-Step Solution

1
Analyze standard IEEE 802.1Q native VLAN default operation.
By default, IEEE 802.1Q trunks transmit traffic belonging to the configured native VLAN without adding a 4-byte 802.1Q header (untagged).
Backward compatibility with non-802.1Q legacy devices requires native VLAN traffic to remain untagged unless configured otherwise.
2
Determine the impact of the global command `vlan dot1q tag native` on egress traffic.
The command alters global switch behavior such that native VLAN frames (VLAN 50 in this scenario) are encapsulated with an 802.1Q header containing the native VLAN ID prior to egress transmission.
Tagging native VLAN traffic prevents VLAN hopping security attacks and ensures consistent identification across switch fabrics.
3
Determine the impact of the global command `vlan dot1q tag native` on ingress traffic.
Because the switch now expects tagged frames for all assigned VLANs on 802.1Q trunk links, any incoming untagged Ethernet frames arriving on the trunk interface are discarded.
Accepting untagged frames when native VLAN tagging is enabled would create ambiguity regarding frame classification.

Key Concept

Global 802.1Q Native VLAN Tagging Behavior
Estimated Time:2m 0s
Rate this question

Topics

Interswitch Connectivity and Trunking (802.1Q)

More questions from this topic