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Layer 2 Problem

 
 
Michael Müller
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      02-21-2006
Hi NG,

i have a strange problem in my network. Sometimes one device (maybe
a workstation or printer or even a server) is unreachable by 50% of all
other devices in the network.A Ping fails and a traceroute stops at the core
router.
They are in different ip segments but all in a broadcast domain. I have to
do an "clear arp-cache"
on my core router in order to get everything fine.
What in hell could that be ?
The log on the router shows nothing weird....

Thx in advance

Michael


 
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ciscodagama@gmail.com
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      02-21-2006
More information about how your network is configured would be needed
to make any educated guesses. Do you have any HSRP in the part of the
network under consideration? Do you have NAT running in conjunction
with HSRP-based load balancing perhaps? A combination like that could
cause problems like this though that only one device becomes
unreachable seems a little weird.

To see more about ARP interaction with HSRP+NAT issues look at

http://www.cisco.com/univercd/cc/td/...4/ftnthsrp.htm

for instance.

Anyway, this is only a wild guess since more detailed information is
missing. Maybe some other posters will have other ideas.

Cisco da Gama
http://ciscostudy.blogspot.com

 
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Michael Müller
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      02-21-2006
Hi,

we do have a cisco 6000 with 4 msfc modules as core router.
hsrp is configured, but no nat routing.

Michael

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
> More information about how your network is configured would be needed
> to make any educated guesses. Do you have any HSRP in the part of the
> network under consideration? Do you have NAT running in conjunction
> with HSRP-based load balancing perhaps? A combination like that could
> cause problems like this though that only one device becomes
> unreachable seems a little weird.
>
> To see more about ARP interaction with HSRP+NAT issues look at
>
>

http://www.cisco.com/univercd/cc/td/...4/ftnthsrp.htm
>
> for instance.
>
> Anyway, this is only a wild guess since more detailed information is
> missing. Maybe some other posters will have other ideas.
>
> Cisco da Gama
> http://ciscostudy.blogspot.com
>




 
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Carlo Poggiarelli
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      02-21-2006

"Michael Müller" <(E-Mail Removed)> wrote in message
news:dtek01$s5$(E-Mail Removed)...
> Hi,
>
> we do have a cisco 6000 with 4 msfc modules as core router.
> hsrp is configured, but no nat routing.
>
> Michael


Hi.

Has MLS been enabled on your MFC/SUPERVISOR?

Let me know


Bye

Carlo


 
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Michael Müller
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      02-22-2006
Hi,

no, MLS is not enabled.
Any idea ??
The Problem drives me crazy....
someone things about a spanning-tree loop....

thx


"Carlo Poggiarelli" <(E-Mail Removed)> wrote in message
news:43fb5389$0$5995$(E-Mail Removed). ..
>
> "Michael Müller" <(E-Mail Removed)> wrote in message
> news:dtek01$s5$(E-Mail Removed)...
> > Hi,
> >
> > we do have a cisco 6000 with 4 msfc modules as core router.
> > hsrp is configured, but no nat routing.
> >
> > Michael

>
> Hi.
>
> Has MLS been enabled on your MFC/SUPERVISOR?
>
> Let me know
>
>
> Bye
>
> Carlo
>
>



 
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Merv
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      02-22-2006
Capture the contents of the arp cache befre and after you do a clear.

Determine if the MAC address for the device that has the reachability
problem is the same
in both cases..

 
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Michael Müller
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      02-22-2006
thats the point....
it IS the same mac adress. But obviously no one knows how to
deliver the packet to the mac. maybe still a loop in the network ?
but why do other clients reach that ip ?


"Merv" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Capture the contents of the arp cache befre and after you do a clear.
>
> Determine if the MAC address for the device that has the reachability
> problem is the same
> in both cases..
>



 
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Merv
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      02-22-2006
a loop would typically affect a lot more than one IP address

so please provide the router & switch details:
1. hardware models
2. IOS / CATOS versions
3. topology for a recent failure case - which switch and port is the
unreachable device connected to, IP addresses htat can reach devices,
IP addresses that cannot reaach device.

 
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Michael Müller
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      02-22-2006
that will be a hell of information...
the network has about 10 routers and over 20 switches....

"Merv" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> a loop would typically affect a lot more than one IP address
>
> so please provide the router & switch details:
> 1. hardware models
> 2. IOS / CATOS versions
> 3. topology for a recent failure case - which switch and port is the
> unreachable device connected to, IP addresses htat can reach devices,
> IP addresses that cannot reaach device.
>



 
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Merv
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      02-22-2006

Perhaps, but if you want to get to the pbottom of this you have to
start characterizing the nature of the problem in more detail

How many subnets (routes) are there in your network?

Does this problem happen on every one of those subnets?

Any there any observable patterns whatsoever - time of day, etc, etc.

Have you kept a lot of every IP address for which this problem has
occurred; if not highly recommend this be done.

How long has this problem being occuring ?

Recent router or switch upgrades, new switches added, topology change,
sighnficant configuration changes ...

If you want to get assistance from this group, this type of info will
be required.

 
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