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Difficulty: Very hardL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch receives an ingress Ethernet frame tagged for VLAN 10 on port GigabitEthernet0/1. The frame contains a source MAC address of 0010.AB12.3456 and a destination MAC address of 0020.CD34.5678. Prior to receiving this frame, the switch MAC address table contains a dynamic entry listing 0010.AB12.3456 on port GigabitEthernet0/2 in VLAN 10, and a static entry listing 0020.CD34.5678 on port GigabitEthernet0/3 in VLAN 10. Port GigabitEthernet0/4 is an active access port configured in VLAN 20, while port GigabitEthernet0/5 is an active 802.1Q trunk port allowing VLANs 10 and 20. Which two actions does the switch perform upon processing this frame?

  1. It updates the MAC address table entry for 0010.AB12.3456 to associate it with port GigabitEthernet0/1 in VLAN 10.Answer
  2. It forwards the frame exclusively out port GigabitEthernet0/3.Answer
  3. C
    It floods the frame out all operational ports across all VLANs, including GigabitEthernet0/4, because the source MAC address moved ports.
  4. D
    It drops the frame immediately because receiving a previously learned dynamic source MAC address on a different port indicates a Layer 2 loop.
  5. E
    It forwards the frame out GigabitEthernet0/3 and replicates a copy out trunk port GigabitEthernet0/5 tagged with VLAN 10.

Answer

The switch updates its MAC address table entry for source MAC 0010.AB12.3456 to port GigabitEthernet0/1 in VLAN 10, and forwards the frame directly out port GigabitEthernet0/3.
When an Ethernet switch receives a frame, it performs learning and forwarding independently. First, it reads the source MAC address 0010.AB12.3456 and updates the MAC address table entry to map to port GigabitEthernet0/1 in VLAN 10. Second, it looks up the destination MAC address 0020.CD34.5678 in the MAC table for VLAN 10. Because a matching entry exists for GigabitEthernet0/3, the switch forwards the frame directly out GigabitEthernet0/3 without flooding.

Step-by-Step Solution

1
Analyze ingress port processing and source MAC learning logic.
The source MAC address 0010.AB12.3456 arrived on GigabitEthernet0/1 in VLAN 10. The switch updates the existing table mapping for 0010.AB12.3456 from GigabitEthernet0/2 to GigabitEthernet0/1.
Layer 2 switches evaluate every incoming frame's source MAC address to build and maintain up-to-date MAC table port mappings.
2
Analyze egress port forwarding logic based on destination MAC lookup.
The destination MAC 0020.CD34.5678 is matched in the MAC table for VLAN 10 pointing to GigabitEthernet0/3.
When a match is found in the MAC table for the frame's destination address in the matching VLAN, known unicast forwarding directs the frame solely to that designated egress interface.

Key Concept

Layer 2 MAC address table dynamic learning, station movement handling, and known unicast forwarding logic.
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Topics

L2 Switching Concepts and MAC Table Operation

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