Switch - Simplified Scenario

Nerd Cafe

let’s revise the tutorial assuming the switch’s MAC address table is already populated with the correct mappings before the ICMP ping starts.

That means:

• The switch already knows:

  • PC1’s MAC → Port 1

  • PC4’s MAC → Port 4

• No flooding will occur at all.

• The switch forwards frames directly based on its existing MAC address table.

Below is the step-by-step tutorial with descriptive titles reflecting that scenario:

Step 1: Pre-Populated Switch Table with All MAC Entries

Description:

• Before the ping starts, the switch already has a complete MAC address table.

• It knows exactly which port each connected PC is on.

Example MAC Table:

MAC Address (PC)	Port
PC1	1
PC2	2
PC3	3
PC4	4

Key point:

No learning or flooding is needed—the switch is ready for direct forwarding.

Step 2: PC1 Sends ICMP Echo Request to PC4

Description:

• PC1 generates an ICMP Echo Request (ping) for PC4 (192.168.1.4).

• The Ethernet frame includes:

  • Source MAC: PC1’s MAC

  • Destination MAC: PC4’s MAC

What happens?

• The switch receives the frame on Port 1.

• Looks up PC4’s MAC in its table → finds Port 4.

• Immediately forwards the frame only to Port 4.

Key learning:

Direct unicast forwarding using existing table.

Step 3: Switch Forwards Request Directly to PC4

Description:

• No broadcast or flooding occurs.

• Only PC4 receives the ICMP Echo Request.

What happens?

• Switch uses its MAC table to unicast the frame to PC4’s port.

• PC2 and PC3 do not see or receive the frame at all.

Key learning:

Efficient delivery—no unnecessary traffic.

Step 4: PC4 Generates ICMP Echo Reply

Description:

• PC4 prepares the ICMP Echo Reply back to PC1.

• Ethernet frame:

  • Source MAC: PC4’s MAC

  • Destination MAC: PC1’s MAC

What happens?

• PC4 sends the reply toward the switch.

• The switch receives the frame on Port 4.

Key learning:

The switch already knows both MACs.

Step 5: Switch Forwards Reply Directly to PC1

Description:

• Switch checks its MAC table for PC1’s MAC.

• Finds it on Port 1.

What happens?

• Immediately forwards the Echo Reply only to Port 1.

• No learning, no flooding, no table updates needed.

Key learning:

Unicast forwarding is streamlined when the table is pre-populated.

Cisco Packet Tracer (Version 6.2.0.0052)

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switch.pkt

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