Re: Strange CDP Observation

Hey Scott!

Interesting points you make here, but I have a few observations to add

At least on my 3560 switch here at home loopguard is not enabled by
default. I have labbed this up an as expected with no loopguard on by
default if I plug a switch into itself everything will be fine -- 1 link
will be the designated port on the segment to itself (the lower port # by
default) and the other end will happily go to blocking.

So with that being said, if we are talking about the same thing here I could
still see how it could be possible. If the switch is plugged into itself,
no loopguard is on (default) AND there is some sort of malfunction like a
uni-directional link there could be serious problems (L2 loop that could
eventually bring the switch to it's knees), or maybe a not so serious
problem like seeing yourself in the output of CDP.

On Sun, Nov 29, 2009 at 3:46 AM, 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?
>
> 1. When you do "show interface" does it say "looped" at all?
> 2. Assuming that regular data moves just fine, it's not a loop
> 3. Back to "show interface" are you seeing it as half duplex? You may
> be if this is a radio device, but that depends on the specifics.
> 4. When you do "show cdp neighbor (int) detail" is the IP address the
> same as yours or different?
>
> I'd sooner think it was a misconfiguration of the other side's switch.
> If all you're doing is L2 stuff on them, you'll probably not notice any
> functional impact. but I've seen more than one customer where EVERY
> switch had identical configuration, including "management IP" and
> "hostname" which makes things REALLY confusing to manage, but fully
> working at L2.
>
> HTH,
>
>
>
>
> *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,
>
> JNCIE-M #153, JNCIS-ER, CISSP, et al.
>
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> Felix Nkansah wrote:
> > Hi Team,
> >
> > One of my field engineers at a remote site has reported a problem to me,
> one
> > which I find rather strange but yet to confirm for myself.
> >
> > A Catalyst 2960 switch at the remote site is linked to a switch at a
> Central
> > site via radio (bridged).
> >
> > However, besides the fact that connectivity tests between the two sites
> are
> > failing, his observation is that when he issues the "SHOW CDP NEI"
> command
> > on the remote C2960 switch, he sees the switch itself appearing as its
> > neighbor rather than the other switch at the central location which is
> > typically the output he expected.
> >
> >
> > I have personally never seen a Cisco device showing itself as it's own
> > neighbor in a 'show cdp neighbor' output.
> >
> > What do you know to be the cause of this problem?
> >
> > Thanks.
> >
> >
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-- 
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Joe Astorino CCIE #24347 (R&S)
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