Re: Strange CDP Observation

Thats precisely what I was thinking also Scott -- STP issue where there was
some sort of malfunction (cable, or something else) whereby both ports think
they are the designated port and go into forwarding. After you mentioned
loopguard I had to dig into it a little more and verify for myself of course
: ) I think we all agree on what some of the possibilities are now and are
all on the same page, which is always a good thing!

On Sun, Nov 29, 2009 at 8:33 AM, Scott Morris <smorris_at_ine.com> wrote:

> Like you, I have seen "looped" in a long time on ethernet, but I haven't
> really had a need to test an ethernet port like that in forever!
>
> Assuming that works in the same fashion that it does on a serial line,
> the router will detect "looped" by virtue of receiving its own
> keepalives back. Not by CDP or any higher level frames.
>
> I don't recall loopguard being turned off (from doing the L2 Tunneling
> entertaining configurations) but I haven't verified that! After you (or
> Joe) mentioned it being turned off, I got to thinking about it though.
> The looping caused there was from a CDP frame being received on a
> DIFFERENT port (same switch though) where it would think of it as an STP
> failure...
>
> I'm still leaning towards duplicate configurations as it's the simplest
> answer to create this difficulty. While it COULD be a more inane
> design-related thing, those are typically more obvious when someone
> attempts to figure out why a problem is occurring.
>
> Just my two cents. :)
>
>
>
>
> *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,
>
> JNCIE-M #153, JNCIS-ER, CISSP, et al.
>
> JNCI-M, JNCI-ER
>
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> Marko Milivojevic wrote:
> > 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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-- 
Regards,
Joe Astorino CCIE #24347 (R&S)
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