Strange Modem

[Rick Stevens]

brad.mugleston at comcast.net wrote:
> On Mon, 22 Aug 2005, Rick Stevens wrote:
> 
> 
>>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.
> 
> 
> 
> Thanks for the help Rick - haven't been able to check things out
> and it started working again.  Got online and checked out the
> comcast web site.  Checked into their forum section and it wasn't
> just my problem but a problem all across the US+.  Talking to
> comcast they said it was my problem but they set out a fix and it
> went away.  Theory is they did a software update on everyones
> modems and it didn't work.....
> 
> It would sure make things eaiser if they weren't lying to all
> their users all the time.

"Lying to protect the guilty and prosecute the innocent (users)".
Fairly common.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-   Never test for an error condition you don't know how to handle.  -
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