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Difficulty: MediumSwitching Concepts and VLANs

A network technician is auditing frame forwarding behavior on an enterprise Layer 2 switch. An Ethernet frame arrives at an 802.1Q trunk interface carrying a tag for VLAN 50. Upon inspecting its MAC address table, the switch finds no entry corresponding to the destination MAC address of the frame. Which TWO of the following actions will the switch execute to handle this frame?

  1. Inspect the source MAC address of the frame and update the MAC address table entry for VLAN 50.Answer
  2. Flood the frame out of all operational interfaces assigned to VLAN 50, excluding the ingress trunk port.Answer
  3. C
    Flood the frame out of all active interfaces across all configured VLANs on the switch.
  4. D
    Strip the 802.1Q tag and route the packet using the Layer 3 destination IP address header.
  5. E
    Remove the 802.1Q header tag and transmit the frame untagged out of native VLAN ports only.

Answer

The switch will record the source MAC address into its MAC address table for VLAN 50 and flood the unknown unicast frame out of all active ports assigned to VLAN 50, excluding the port on which it arrived.
When a frame reaches a switch interface, the switch performs source MAC learning and records the association between the MAC address, ingress port, and VLAN 50. If the destination MAC address is absent from the CAM table, the switch executes unknown unicast flooding, transmitting the frame out of all active ports assigned to VLAN 50 except the receiving ingress port.

Step-by-Step Solution

1
Analyze ingress processing for source MAC address learning.
The switch reads the source MAC address and 802.1Q VLAN ID (VLAN 50) to update or add an entry in its CAM/MAC address table for the ingress port.
MAC address learning happens on incoming frames regardless of whether the destination address is known.
2
Evaluate destination lookup for frame forwarding.
Since the destination MAC address is missing from the MAC address table, the frame is classified as an unknown unicast frame.
Unknown unicast traffic requires flooding to ensure delivery to the intended destination host.
3
Apply VLAN broadcast domain containment rules.
The frame is flooded to all interfaces belonging to VLAN 50, omitting the ingress interface.
VLANs act as distinct Layer 2 broadcast domains, restricting unknown unicast flooding strictly within the specified VLAN boundary.

Key Concept

Layer 2 MAC Table Learning and Unknown Unicast Flooding within VLAN Boundaries
Estimated Time:1m 30s
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Topics

Switching Concepts and VLANs

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