The Switch Learns Too

Lesson 8 of 35 in Coddy's LAN Labs course.

A switch does not know addresses when it is switched on. It learns: every frame that arrives on a port teaches the switch "this MAC lives behind this port". The result is its MAC table, and it is what lets a switch deliver a frame to exactly one port instead of flooding every port like an old hub.

netlab show mac SWITCH displays the table. Empty before traffic; after a ping between two hosts it holds both MACs, each mapped to the port the host is plugged into.

Challenge

Beginner

Ping pc2 from pc1, then show sw1's MAC table.

Expected output:

pc1 -> pc2: reply from 192.168.1.20 (ttl=64)
PORT   MAC                LEARNED-FROM
port1  02:c0:dd:aa:ec:b3  pc1
port2  02:c0:dd:a0:9f:64  pc2

Try it yourself

Terminal

All lessons in LAN Labs

1Meet the Lab

Your First Lab Cables Wires Are Not Enough Recap - Build a Lab

4Routers and Gateways

Two Networks The Default Gateway Both Sides Need a Way Back Routing Tables Recap - Connect the Floors

7Growing the Office

The Plan Sales Comes Online Engineering Joins Gateways Everywhere Recap - The Printer

2Addresses on the LAN

Giving a Host an Address The First Ping What ARP LearnedThe Switch Learns TooRecap - First Conversation

5Troubleshooting I

The Unplugged Host The Missing Gateway The Typo in the Mask Recap - Two Faults

8Capstone - Fix the Branch

Survey the Damage Fix the Address Clash Masks and Gateways Recap - Prove It Works

3Masks Matter

Wrong Mask, No Reply Same Switch, Other Subnet Let check Do the Looking Recap - Diagnose the Pair

6Troubleshooting II

Two Devices, One Address The Network Address Trap Gateway on the Wrong Subnet Recap - The Silent Router

Practice on your own: Terminal playground