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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

Switch SW2 receives an ingress Ethernet frame on interface GigabitEthernet0/5 configured as an access port in VLAN 20. The frame contains a source MAC address of 0010.ab12.3456 and a destination MAC address of 0010.ab99.8877. A review of SW2's MAC address table reveals that neither MAC address currently exists in the table for VLAN 20. Which two actions does the switch take upon processing this frame? (Select two.)

  1. Records MAC address 0010.ab12.3456 associated with interface GigabitEthernet0/5 in the VLAN 20 MAC address table.Answer
  2. Floods the frame out all active interfaces assigned to VLAN 20, excluding interface GigabitEthernet0/5.Answer
  3. C
    Floods the frame out all operational interfaces across all configured VLANs on the switch.
  4. D
    Drops the frame immediately because the destination MAC address is not present in the MAC address table.
  5. E
    Generates and sends an ARP request from interface GigabitEthernet0/5 to resolve the IP address corresponding to MAC 0010.ab99.8877.

Answer

The switch records the source MAC address 0010.ab12.3456 dynamically under GigabitEthernet0/5 in VLAN 20, and floods the unknown unicast frame out all active ports in VLAN 20 except the ingress port GigabitEthernet0/5.
When a switch receives a frame, it performs two distinct steps: learning and forwarding. First, it reads the incoming frame's source MAC address and associates it with the receiving port and VLAN in its MAC address table. Second, it checks the destination MAC address. Because the destination is not in the table, it treats the frame as unknown unicast and floods it out all active ports in the same VLAN except the receiving port.

Step-by-Step Solution

1
Inspect the source MAC address of the ingress frame.
The source MAC address 0010.ab12.3456 is learned on port GigabitEthernet0/5 for VLAN 20 and added/updated in the MAC address table.
Switches build and refresh their Layer 2 forwarding table using incoming source MAC addresses.
2
Inspect the destination MAC address of the ingress frame.
The destination MAC address 0010.ab99.8877 is not found in VLAN 20's MAC address table, classifying it as unknown unicast.
A lookup in the MAC table determines whether known unicast forwarding or unknown unicast flooding is required.
3
Forward the unknown unicast frame according to Layer 2 rules.
The frame is flooded to all active ports belonging to VLAN 20, except GigabitEthernet0/5.
Flooding ensures delivery to the unknown destination while avoiding sending traffic back out the receiving interface.

Key Concept

Layer 2 Switch Learning and Unknown Unicast Flooding
Estimated Time:1m 30s
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Topics

L2 Switching Concepts and MAC Table Operation

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