I have a cisco 3845 configured with a HWIC-2T cable modem card. the router is connected to a 3750e 48 port switch. the switch contains 3 vlan interfaces:
management : 192.168.1.254/24
data: 192.168.10.254/24
voice: 192.168.20.254/24
the 3750 is configured to hand out dhcp. the 3845 is trunked to the siwtch and the router contains 3 sub interfaces (192.168.1.1, 192.168.10.1, 192.168.20.1).
I also have a wireless lan controller in the 3845 with a few 1140 access points connected.
Here is my problem:
If i connect wirelessly the clients connect, recieve an ip address via the switch and are fine, even on the data vlan.
my phones are also working fine on the voice vlan, they recieve an ip and are fine.
if i plug directly into a data vlan port (most ports are configured access 10, voice 20) i recieve an ip address on 192.168.100.x/24 subnet (i expected .10). this subnet is not configured anywhere on my network. i verified this with a show run | i 192.168.100 on all devices. after some head scratching i did a show tech and discovered that the cable-modem hwic's internal (and non configurable) ip address is 192.168.100.1. the cisco 3845 seems to have also automatically created a 192.168.100.2 address. it does not show up in the configuration but is pingable on the network, and if i telnet to 192.168.100.2 it is my router. Again though there is no configuration entry for either address.
What is extremely odd is this was not always the case. If i assign an address on the data network statically there is no problem. I recently upgraded the ios and i thought maybe this caused the problem, but a downgrade did not resolve the symptoms. i will post my configurations sunday, but since the configs do not actually have these ip addresses in them its not very telling.
i have a cisco 1841 that also has a cable modem hwic in it (different site). i have the internal network configured as 192.168.30.0/24 with no vlans. i have confirmed that even on this network 192.168.100.1 is present, and the router is reachable via 192.168.100.2 (again not in the configuration so this seems to be a standard thing with the cisco cable hwics). i have never experienced any issues with these 192.168.100.x addresses being recieved by clients on this network though.
i am going to close this question. it turns out that the issue was a recently added cable tv hd/dvr box in one of the management offices. its strange to me that these are even capable of functioning as a dhcp server. i am unable to find any network settings on the device but it was definetely the culprit. it seems that it was handing out 192.168.100.x address and it was just a coincidence that the cable modem also had that internal address, unfortunately i was thrown off by the fact that when i telneted into 192.168.100.2 it was my router... and from there i saw the internal ip of the WS-X45-SUP7-E cable modem which was 192.168.100.1