Discussion:
Problem with Windows7 Manual IP
(too old to reply)
t***@gmail.com
2014-05-27 11:58:41 UTC
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Hello,

We have a Cisco 2901 with multiple WICs and three of the Routing Interfaces are connected to the same single dumb switch.

Obviously the interfaces should not normally be connecting to the same switch, but that's how our system is currently designed.

The odd thing is when a Windows PC (no IP yet) and another device (device A,for example IP 10.10.10.1) are both connected to this same dumb switch, when we try to configure an IP on the Windows PC, the PC responds with IP address conflict. We are sure there is no IP conflict, but the message comes up everytime.

Even though we can try any random IP on the Windows PC, it always responds with IP conflict.

If I remove the three network cables from the router interfaces, with just the Windows PC and device A connected, I can configure the Windows PC IP with no problem and communicate with device A. Ever after putting the cables back into the router, everything seems fine at that point.

So the problem seems to only appear when trying to set an IP on the PC, but once the IP is set, all seems well. I'm certain it's not related to a true IP conflict, but maybe something in the way Windows detects IP conflicts?

Maybe PC having the same MAC address across multiple interfaces causes this????

Does anyone know why the PC would think there is an IP conflict when trying to set the IP?

Very curious...

Thanks in advance!
h***@gmail.com
2016-09-27 21:27:16 UTC
Permalink
Post by t***@gmail.com
We have a Cisco 2901 with multiple WICs and three of the
Routing Interfaces are connected to the same single dumb switch.
Obviously the interfaces should not normally be connecting to the
same switch, but that's how our system is currently designed.
The odd thing is when a Windows PC (no IP yet) and another
device (device A,for example IP 10.10.10.1) are both connected to
this same dumb switch, when we try to configure an IP on
the Windows PC, the PC responds with IP address conflict.
We are sure there is no IP conflict, but the message comes up everytime.
Even though we can try any random IP on the Windows PC, it always
responds with IP conflict.
(snip)
Post by t***@gmail.com
Maybe PC having the same MAC address across multiple
interfaces causes this????
Does anyone know why the PC would think there is an IP
conflict when trying to set the IP?
Well, this is a little old, but I will reply anyway.

The test for duplicate IP address is to ARP for your own address, and see
if anyone else replies. Routers should not forward ARP, though they might
have proxy-ARP turned on. If proxy-ARP was on, and I don't think it should
be, the ARP could go out another interface to the same net, and surprise
the Windows PC.

Use ethernet sniff software, such as tcpdump, and see what packets
are floating around.

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