Switching: how a switch learns where everyone is

The idea

Alice, bob and carol each have one cable to officeSwitch, a Switch. You will watch it learn which of its ports each MAC address sits behind, flood what it has not learned yet, forget, and fill up.

Steps

1. A switch is ports and a table

   Each cable ends in one port of officeSwitch, named after the device at the other end. List them first: everything the switch learns, it files by port.

   officeSwitch:~# bridge link show
   officeSwitch:~# ip -br addr show master br0

2. Learning from source addresses

   The switch's table starts empty. Alice pings bob; then list what the switch learned, since that table is how it later sends a frame to one port instead of all.

   alice:~# ping -c1 -W2 10.0.1.11
   officeSwitch:~# bridge fdb show br br0 dynamic

3. Unknown destination: flood it

   The table is emptied again. Alice pings bob three times while carol records every ping that reaches her card, to see what a switch does with a frame it cannot place yet.

   alice:~# ping -c3 10.0.1.11

4. Aging: forgetting who went quiet

   Devices unplug and move, so the switch forgets an entry that stays quiet for its aging time, 20 seconds here; else it would keep sending to a port nobody is behind. Read the table with its timers.

   officeSwitch:~# bridge -s fdb show br br0 dynamic
   officeSwitch:~# bridge -s fdb show br br0 dynamic

5. A full table turns the switch into a hub

   This switch's table holds 16 entries, and anyone plugged in can fill it. Read the switch's counter, then ping bob from alice while carol records, to see what a switch does when it cannot learn.

   officeSwitch:~# ip -d link show br0
   alice:~# ping -c3 10.0.1.11
   alice:~# ping -c3 10.0.1.11

6. A switch is not a hop

   A Router is a hop on the way: it lowers the TTL and answers for it. Trace the way from alice to bob, to see whether the switch takes part in IP at all.

   alice:~# traceroute -n 10.0.1.11

Run these steps on real machines