[OT] Cable modem problem solved with hub

[O Polite <ol1@v10a.com>]

I posted this on comp.os.linux.networking to no avail. 

-----------------------------
I've just managed to solve a problem with my cable modem connection.

The ISP gave me a Motorola Springboard Cable Modem (let's call it CM)  a
Netgear Router/Firewall (let's call it R).
The default configuration is to use CM - R - LAN (no custom firewall)

However I wanted to run an http server and some other stuff on the
inside 
and had already set up a linux iptables firewall to do all the right 
portforwarding. This had worked nicely with the old ISP and I made a
deal 
with the new ISP that I could connect my linux firewall (FW) directly to
the CM and be assigned a static IP number. (Actually the firewall has to
make a dhcp request, but it will  always be assigned the same IP number 
based on it's MAC address) 

The first thing I did when the CM and R arrived was to throw out the R.
I set up 

CM - FW - LAN

However I ran into trouble. The connection was very flaky. It would drop
every hour or every ten minutes forcing me to power cycle the CM or 
ifupdown the external interface on the FW.

Then I tried 
CM - R - LAN 

It worked fine. (But this was not an option because it wouldn't give me 
static IP and the right portforwardning)

So I figured there was something wrong with my FW. I did a lot of kernel
reconfigurations.

CM - FW - LAN   still didn't work

Then I tried.

CM - R - FW - LAN

Worked like a clock.

Now I start thinking that there is something wrong with the dhcp request
from the FW and decide to do some really brutal troubleshooting.


I set up CM - HUB - FW - LAN
               |
              PS

PS in the diagram is a box running a packet sniffer.

I never really get into analyzing the traffic because suddenly the 
connection is working flawlessly days on end.

I remove the hub to get back to 

CM - FW - LAN

and the connection starts dropping again.

I go back to 

CM - HUB - FW - LAN  (no PS this time)

and everything is working.


This is were I am now. Everything is working the way I want and I don't 
know why.

The theory now is that something is broken on the ethernet level,
something 
that won't show up in an IP packet sniffer.
Either the CM or the NIC on my FW is flaky and they can't cope with 
oneanother. The hub sorts it out by not reproducing signals exactly as
they 
come in. The hub is somehow mending bad ethernet packets. Note that 

CM - R - FW - LAN 

worked fine too. So my theory also states that the router does the same 
ethernet packet mending as the hub.


Is this all plausible? Any ethernet wizards out there that can help me
get 
to the bottom of this?

oivvio polite


--------
Profound yet funny signature