The Switch Learning Process

When Computer A sends data to Computer D:

  1. MAC Learning

    • The switch looks at the Source MAC (A) and the physical port it arrived on (Port 1).
    • Stores entry in MAC Address Table/CAM table/Frowarding Database/Bridge Table
    • Switch learns: MAC A ā†’ Port 1

  2. Forwarding Decision

    • Switch checks Destination MAC (D)
    • If D is in the table
      • Send frame only to Dā€™s port (Unicast)
    • If D is NOT in the table
      • Flood frame to all ports except the incoming one
      • This is called Unknown Unicast Flooding
    • After D Replies
      • Switch learns: MAC D ā†’ Port 3

Gateway Decision

  • If (Source IP & Mask) == (Destination IP & Mask) then Local
  • If NOT equal : Send to Gateway

1. A to D (Local Traffic)

alt text alt text

  • A (192.168.1.10) checks 192.168.1.13 with its subnet mask (/24).
  • Result: Same network (192.168.1.0/24).

Network Layer (IP Layer)

  • Source IP: 192.168.1.10
  • Destination IP: 192.168.1.13

Data Link Layer (MAC Layer)

  • Source MAC: A
  • Destination MAC: D

Summary Table : Switch Communication

Device Layer Action Result
Switch Learns MAC A is on Port 1 Forwards to destination port(s).
Computer B Checks MAC address: Matches B? No DROP
Computer C Checks MAC address: Matches C? No DROP
Computer D Checks MAC address: Matches D? Yes ACCEPT

2. A to 8.8.8.8 (Remote Traffic)

alt text alt text

alt text alt text

Network Layer (IP Layer)

  • Source IP: 192.168.1.10
  • Destination IP: 8.8.8.8

Data Link Layer (MAC Layer)

  • Source MAC: A
  • Destination MAC: E (Router MAC)

Because 8.8.8.8 is remote, the frame must go to the Default Gateway (Router).