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

[h bagade]

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?!

I've also tested Cisco router connection on different systems with
different OSes. On Win systems, when I disable the Ethernet card, router
detects it at the time but on FreeBSD systems, when I set interface down,
the router shows Line Protocol as up!
I tried to capture packets with Wireshark on Win system to find out if any
packet is sent out before the interface is disabled but I see no packet! I
thought maybe it stops capturing on interface and then send the supposed
packet out!

I really confused by what I saw in different scenarios! I need help to fix
it up.

On Sat, Jan 5, 2013 at 11:15 PM, Jay Hennigan <jay at west.net> wrote:

> On 1/5/13 3:44 AM, h bagade wrote:
> > 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!
>
> Some of this depends on the layer 2 protocol (Ethernet vs. DS-3 for
> example) but in most cases there isn't any detectable difference between
> the remote end being administratively shut down and a failure of the
> interconnecting medium.
>
> The exception is that in some metro ethernet scenarios you can use OAM
> to capture dying-gasp, error disable, or shutdown events.  It isn't a
> periodic poll, but rather like a one-time "Going down now!", your
> scenario 1.
>
> > 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!
>
> The router will absolutely detect the lack of line protocol and carrier
> and flag the link as down but this would be the case whether the remote
> side is administratively shut down or the cable is just unplugged.
>
> > 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!
>
> See:
>
> http://www.cisco.com/en/US/docs/switches/metro/me3400/software/release/12.2_46_se/configuration/guide/swoam.pdf
>
> --
> Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
> Impulse Internet Service  -  http://www.impulse.net/
> Your local telephone and internet company - 805 884-6323 - WB6RDV
>