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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

An enterprise Catalyst switch has recently rebooted, and its MAC address table is entirely empty. Host Workstation-1 is connected to port FastEthernet0/12 in VLAN 10 and sends a unicast frame addressed to Server-1 (MAC address 0050.56a1.1a02). The frame enters FastEthernet0/12 carrying a source MAC address of 0050.56a1.1a01. How will the switch process this incoming Ethernet frame?

  1. The switch records MAC address 0050.56a1.1a01 mapped to port FastEthernet0/12 in VLAN 10, and floods the frame out all operational ports assigned to VLAN 10 except FastEthernet0/12.Answer
  2. B
    The switch drops the frame immediately because the destination MAC address 0050.56a1.1a02 does not exist in the MAC address table.
  3. C
    The switch populates its MAC address table with both the source MAC on FastEthernet0/12 and the destination MAC on its expected egress interface.
  4. D
    The switch updates its MAC address table with the source MAC address and floods the frame out all operational ports across all VLANs on the switch, including FastEthernet0/12.

Answer

The switch records MAC address 0050.56a1.1a01 mapped to port FastEthernet0/12 in VLAN 10, and floods the frame out all operational ports assigned to VLAN 10 except FastEthernet0/12.
When a Layer 2 switch receives an Ethernet frame, it first inspects the source MAC address and associates it with the ingress port and VLAN in its MAC address table. Next, it looks up the destination MAC address. Because the MAC table is empty, the destination MAC is unknown. The switch performs unknown unicast flooding by forwarding the frame out every active port assigned to the same VLAN, excluding the port on which the frame arrived.

Step-by-Step Solution

1
Examine the ingress frame's source MAC address and ingress port.
The source MAC address is 0050.56a1.1a01 on ingress port FastEthernet0/12 (VLAN 10).
Layer 2 switches dynamically build their MAC address table by inspecting the source MAC address of every incoming Ethernet frame.
2
Update the MAC address table entry for VLAN 10.
An entry mapping 0050.56a1.1a01 to FastEthernet0/12 in VLAN 10 is added or refreshed.
Learning source MAC addresses ensures future frames destined for Workstation-1 can be forwarded directly via unicast.
3
Lookup the destination MAC address 0050.56a1.1a02 in the MAC address table.
The destination MAC address is not found (unknown unicast).
The switch MAC address table was cleared after reboot and has not yet observed traffic originating from Server-1.
4
Determine the forwarding action for the unknown unicast frame.
The frame is flooded out all active switch ports belonging to VLAN 10, except the ingress port FastEthernet0/12.
Flooding guarantees delivery to the destination host while restricting traffic within the VLAN boundary and preventing hairpinning back out the ingress port.

Key Concept

L2 Switching Concepts and MAC Table Operation
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Topics

L2 Switching Concepts and MAC Table Operation

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