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Difficulty: MediumSwitching Concepts and VLANs

A network technician connects a new workstation to Port 4 on a Layer 2 switch. Port 4 is configured as an access port in VLAN 30. The workstation immediately sends a broadcast ARP request to discover its default gateway. The switch currently has active access ports in VLAN 10 and VLAN 30, as well as an 802.1Q trunk port carrying both VLANs (with VLAN 1 as the native VLAN). Which of the following actions will the switch take upon receiving this frame?

  1. Forward the frame to all active access ports in VLAN 30 and encapsulate it with an 802.1Q tag for VLAN 30 across the trunk port.Answer
  2. B
    Flood the frame out all active access ports in both VLAN 10 and VLAN 30 to ensure the default gateway receives the ARP request.
  3. C
    Inspect the IP payload and route the packet directly to the default gateway interface residing in VLAN 10.
  4. D
    Drop the frame at the ingress port because untagged broadcast traffic cannot traverse an 802.1Q trunk port.

Answer

The switch forwards the broadcast frame out all active access ports assigned to VLAN 30 and tags the frame with an 802.1Q header for VLAN 30 before transmitting it across the trunk port.
VLANs partition a physical switch into distinct Layer 2 broadcast domains. When an untagged broadcast frame enters an access port assigned to VLAN 30, the switch floods the frame to all local active interfaces belonging to VLAN 30. Furthermore, when forwarding the frame across an 802.1Q trunk link where VLAN 30 is non-native, the switch inserts an 802.1Q tag header containing the VLAN 30 ID so the adjacent switch maintains proper VLAN isolation.

Step-by-Step Solution

1
Identify the ingress port's VLAN assignment and frame type.
The frame enters Port 4 (assigned to VLAN 30) as an untagged Layer 2 broadcast frame destined for ffff.ffff.ffff.
Traffic entering an access port is implicitly associated with that port's configured access VLAN.
2
Determine the Layer 2 forwarding domain boundaries.
The broadcast domain is restricted exclusively to interfaces belonging to VLAN 30.
VLANs logically segment a physical switch into isolated Layer 2 broadcast domains.
3
Apply 802.1Q trunking rules for outgoing interfaces.
The frame is flooded to all active local access ports in VLAN 30 and sent out the trunk port with an added 4-byte 802.1Q header specifying VLAN 30.
802.1Q trunk links carry traffic for multiple VLANs by appending VLAN identification tags to non-native VLAN frames.

Key Concept

VLAN Broadcast Isolation and 802.1Q Trunk Tagging
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Topics

Switching Concepts and VLANs

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