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

[Jay Hennigan]

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/
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