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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch receives an Ethernet frame on interface GigabitEthernet0/12 configured as an access port in VLAN 30. The incoming frame has a source MAC address of 00e0.8b11.2233 and a destination MAC address of 00e0.8b99.8877. The switch's MAC address table currently contains no entries for interface GigabitEthernet0/12 and does not contain the destination MAC address. Which two actions does the switch perform upon receiving this frame? (Select two.)

  1. The switch records the source MAC address 00e0.8b11.2233 associated with interface GigabitEthernet0/12 in its VLAN 30 MAC address table.Answer
  2. The switch floods the frame out all operational ports assigned to VLAN 30, excluding the receiving interface GigabitEthernet0/12.Answer
  3. C
    The switch floods the frame out all active interfaces across all configured VLANs on the switch.
  4. D
    The switch drops the frame immediately because the destination MAC address is absent from the MAC address table.
  5. E
    The switch forwards the frame out GigabitEthernet0/12 back to the sending host to request ARP resolution.

Answer

The switch records the source MAC address 00e0.8b11.2233 associated with interface GigabitEthernet0/12 in its VLAN 30 MAC address table, and floods the frame out all operational ports assigned to VLAN 30, excluding the receiving interface GigabitEthernet0/12.
When a switch receives a frame, it performs two distinct steps: MAC learning based on the source MAC address, and frame forwarding based on the destination MAC address. First, it records the source MAC address 00e0.8b11.2233 and its associated ingress port GigabitEthernet0/12 in the MAC table for VLAN 30. Second, because the destination MAC address 00e0.8b99.8877 is not in the MAC table, the switch handles it as unknown unicast traffic by flooding it to all other active ports configured in VLAN 30, excluding ingress port GigabitEthernet0/12.

Step-by-Step Solution

1
Examine source MAC address processing.
The switch reads source MAC 00e0.8b11.2233 on ingress interface GigabitEthernet0/12 (VLAN 30) and updates its MAC address table entry for VLAN 30.
MAC address table learning is performed based on the source MAC address of incoming frames.
2
Examine destination MAC address lookup.
Because destination MAC 00e0.8b99.8877 is not in the MAC address table for VLAN 30, the frame is classified as unknown unicast.
Unknown unicast frames must be flooded within the VLAN scope.
3
Determine forwarding interface scope.
The frame is forwarded to all active ports in VLAN 30 except ingress port GigabitEthernet0/12.
Flooding rules dictate forwarding to all ports in the same broadcast domain except the receiving port.

Key Concept

L2 Switching MAC Address Table Learning and Unknown Unicast Flooding
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Topics

L2 Switching Concepts and MAC Table Operation

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