[c-nsp] Strange CDP Observation

[Marko Milivojevic]

On Sun, Nov 29, 2009 at 08:46, Scott Morris <smorris at ine.com> wrote:
> While not normal, think about what makes it occur.
>
> If it REALLY WAS your own CDP frame, then your link should be down due
> to loopguard.  Even with a hub there, a hub is a repeater, so is ti
> feasible to see your own stuff?

Well, not really - otherwise it would be pretty hard to do "external
loopback" solutions, as well as testing links using Ethernet loopback
- which is still doable. I haven't seen "looped" status on Ethernet
for a long time.

I think there are several explanations for this problem:

1. Most obvious is that there is a simple loop somewhere. That needs
to be investigated in MW configuration.

2. Don't forget that CDP is Cisco proprietary protocol. Other
equipment usually doesn't have any special processing for these
multicast frames. Furthermore, I have seen really bad multicast
implementations where multicast frames would be flooded on all ports -
including the one it was received from. This is what could be
happening to you - MW is simply returning you back your multicast
traffic. It *shouldn't* but it does.

3. You could be having some more creative problem, like Y-loop (you
have A-to-B communication, but you are also getting this traffic back
to A - A-to-A).

In any case, I would focus my investigation on what's going on in the
microwave part of the setup.

--
Marko Milivojevic - CCIE #18427
Senior Technical Instructor - IPexpert

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