How does switch check line protocol ?

[carl_townshend]

Hi guys

very basic question, how does the switch check the layer 1 status of the switch is up? I.e line protocol

does it listen for data or sense a voltage of some sort?

xheers

[balaji.bandi]

yes kind of electric signal on layer 1.

if you like to go deep dive you need to read IEEE / RFC

below wiki help you ;

https://en.wikipedia.org/wiki/Ethernet_physical_layer

[MHM Cisco World]

all L2 protocol have such as hello message, both end exchange this hello message with some detail about L2 protocol in it end, 
if the L2 protocol is mismatch then the L2 is down as show in log below 
I use two router and use one end using PPP and other use HDLC and since this two protocol is mismatch then the L2 is down.