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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

An engineer is troubleshooting traffic flow on switch SW-BuildingA. A connected device sends an Ethernet frame that arrives on interface GigabitEthernet0/4. The frame has a source MAC address of 0050.56a1.1a2b and a destination MAC address of 0050.56b2.2c3d. When the switch performs a lookup for the destination MAC address in its MAC address table, it identifies an existing dynamic entry mapping 0050.56b2.2c3d to GigabitEthernet0/4. Which action will the switch perform for this frame?

  1. Learns or updates the source MAC address entry on GigabitEthernet0/4 and filters (drops) the frame.Answer
  2. B
    Floods the frame out all active interfaces assigned to the VLAN except GigabitEthernet0/4.
  3. C
    Forwards the frame back out interface GigabitEthernet0/4 to reach the destination host.
  4. D
    Generates a Layer 2 ICMP redirect frame back to the sender host and drops the original frame.

Answer

The switch updates or records the source MAC address entry on the ingress port (GigabitEthernet0/4) and filters (drops) the frame without forwarding it.
When a Layer 2 switch receives a frame, it reads the source MAC address to update its MAC address table. Next, it looks up the destination MAC address. If the destination MAC address maps to the exact same port on which the frame arrived, the switch filters (drops) the frame because the destination node is already on that local segment and does not need switch forwarding.

Step-by-Step Solution

1
Examine the ingress interface and source MAC address of the incoming Ethernet frame.
The switch records or updates the MAC address 0050.56a1.1a2b associated with interface GigabitEthernet0/4 in the MAC address table.
Layer 2 switches constantly learn and refresh source MAC address table bindings upon receiving incoming frames.
2
Perform a destination MAC address table lookup for 0050.56b2.2c3d.
The lookup matches an existing dynamic entry pointing to interface GigabitEthernet0/4.
The switch needs to determine which egress port corresponds to the destination host.
3
Compare the egress interface identified in the MAC table lookup with the ingress interface of the frame.
The egress interface (GigabitEthernet0/4) matches the ingress interface (GigabitEthernet0/4).
When ingress and egress ports match, the frame is already present on that physical network segment.
4
Apply frame forwarding/filtering rules based on matching ports.
The switch filters (drops) the frame.
Forwarding the frame back out the ingress port is unnecessary and would cause duplicate frames on a shared Layer 2 segment.

Key Concept

Frame Filtering and MAC Address Table Lookup Rules
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Topics

L2 Switching Concepts and MAC Table Operation

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