[c-nsp] cisco interface shutdown detection, how is possible?

[h bagade]

Hi all,

I was wondering how Cisco routers could detect the directly connected
interface at the other end is shutdown!

there are two general possibility on my point of view:
1- the other device is sending special information before shutting down the
interface.
2- there are some method of polling which is done periodically and based on
the answer, the router detect the interface is up or no!

As Cisco router is not able to detect the interface shutdown on the other
side when connected to some other device, not Cisco like unix systems, it
seems, it has some sort of protocol for detection which is number 2 of
above guesses!

could you please help me on this? Or provide me a scenario witch I could
find out if any packet is transmitted between Cisco routers to inform the
interface shutdown!