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:
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)
Last updated