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

[Phil Mayers]

On 01/07/2013 06:32 AM, h bagade wrote:
> Can I conclude from all discussions above that the ethernet protocol
> support a feature named "dying gasp" which inform the other end that it is
> going to shutdown? It seems that it works when we intentionally try to
> shutdown an interface but when there is a failure on layer2 connection it
> couldn't help?!

No. The spec contains this feature, but virtually nothing supports it.

Link detection on ethernet is actually a slightly subtle area, but if:

  1. You have autoneg enabled, and
  2. The devices are *DIRECTLY* connected

...you can be sure it will work IF THE LINK GOES DOWN.

There's a lot of confusion in your emails, but what I think you're 
asking is:

if you connect two Cisco devices, how is one able to detect the other 
doing "shutdown"?

...the answer is that on a Cisco, "shutdown" actually disables the 
physical layer, and the other Cisco detects this. In your case, the 
other device (BSD) is not doing this, so this doesn't happen.

There's no magic - if the light/electricity goes away, the Cisco will 
see "line down". If it doesn't, it won't.