Strange Modem

[Rick Stevens]

brad.mugleston at comcast.net wrote:
> On Fri, 19 Aug 2005, Rick Stevens wrote:
> 
> 
>>brad.mugleston at comcast.net wrote:
>>
>>>I just built a file server using an old Pentium II with FC2 but
>>>it's got a 180G hard drive on it (for home use).  When I hook it
>>>into my home network everything works great - the switch assigns
>>>it an IP address and it seems to work fine.  BUt after awhile the
>>>modem starts rebooting.  If I unplug the network cable from the
>>>comuter the modem goes back to normal.  I've tried two different
>>>NIC's in the computer and they both do the same thing.
>>>
>>>Any ideas?
>>
>>Uh, serial modem or PCI-based?  If it's PCI, have you looked at the IRQ
>>assignments between the modem and the NICs?  (lspci -v)
>>
> 
> 
> Sorry - it's a Cable Modem - RCA - goes to my Motorola WR850G
> Router.  I'll have to check the dmesg and interrupts - get back
> ot you later on the rest of it.

Oh.  Hmmmm.  So, I take it that your layout is something like:

            -------------      ------------------
--cable-->| cable modem |--->|WAN router/switch |
            -------------        port 1   port 2
                               ------------------
                                  ^         ^
                                  |         +--System 2
                                  +---System 1

If so, then SOMEONE on your network is probably asking the modem to
fetch a new DHCP connection from your ISP.  You'd need to watch the
TCP traffic to know for sure.  You could use something like:

     tcpdump dest host local-ip-address-of-cable-modem

and that ONLY if you can get the monitoring machine and the cable modem
on the same cable segment (insert a hub--NOT a switch--into the WAN
connection between your router and modem and plug your machine into
that hub as well).

If that's the case, you can fix that by setting the cable modem's WAN
port to a different subnet.  E.g. my cable modem is 192.168.100.1/16,
while my switch uses 192.168.0.1/24 on all ports EXCEPT the WAN port.
The DHCP server on my switch also uses 192.168.0.0/24 in its pool.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-      To err is human, to forgive, beyond the scope of the OS       -
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