Lessons · Network switches · spotting a loop
What a storm looks like from the doorway
Every light on the switch blinking in unison, and a network that is unusable rather than merely slow, is a broadcast storm: the same frames going round a loop for ever and arriving at every port at the same instant.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
It is the fastest diagnosis on this track and you make it from the doorway, before logging in. Knowing it saves the twenty minutes otherwise spent on whichever one PC was reported first.
How to think about it
Read the pattern rather than the brightness. Ordinary traffic makes lights blink raggedly, each port on its own business. In unison means one thing arriving everywhere at once, and the usual cause is a patch lead with both ends in the same room.
Worked example
enableIf the console still answers. A switch in a storm can be too busy to type into, and that is itself information.
show interfaces statusWhich ports are actually connected. A loop needs two, and they are often two that nobody meant to join.
show mac address-tableOne address appearing on a port it has no business being on is the loop, seen from the inside.
configure terminalSwitch(config)#.
interface gi0/2The likelier half of the pair.
shutdownBreak the loop first and investigate second. A storm does not get easier to diagnose while it is running.
endSwitch#.
Your turn
Find out which ports are connected to something.
show interfaces
Console into a switch
The trap
Rebooting the switch. It comes back, the loop is still patched in, the storm returns in seconds, and you have now also lost everything that would have told you where it was.