[c-nsp] Port Question

[Aaron]

Hey Guys,

 

This is a really basic question and I should know the answer. I am just
pondering over loop guard and UDLD and take this for example:

 

Two switches are connected via a pair of fibre (Tx at one end RX at the
other). Now let's say the TX path from switch1 (RX on switch2) fails or is
disconnected but the other fibre is still operational. I would have thought
that logically Switch 2 should see that it is no longer receiving "light" on
the cable and bring the port down? 

 

Obviously this is not the case. Therefore my question is what determines if
a port is physically "UP"? What physical checks are done by the switch port
and how does it determine if media is attached? Is a frame received over the
fibre initially which brings the port up but a subsequent failure will go on
undetected?

 

Any reference links would be great J

 

Thanks group.

 

Aaron.