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From: "itallushrt" <itallushrt@hotmail.com>
Subject: Re: AT&T Cable uses Mac ID
Date: Tue, 5 Feb 2002 11:07:56 -0500
References: <20020205.083753.1139901474.1418@math.northwestern.edu> 

I am a NOC Engineer for a Cable modem ISP and can tell you what is going on
here is not rocket science. AT&T, just like 99.9% of the rest of cable modem
providers, uses Cisco UBRs (Universal Broadband Routers) as their CMTS
(Cable Modem Termination System).  It is essentially a 7200 series router
with MC16x Cable Blades - each consisting of 5 upstream ports and 1
downstream port. Your modem will allow you to "surf" with any IP you assign
to it, granted its in use and routed on the network your on, thus that is
why you were still working with the old IP.  Second,  the subject line "AT&T
Cable uses MAC ID" is true for any cable modem system. The modems are
authorized, deauthorized, provisioned, etc depending on the HFC MAC ID of
the modem using Cisco's CNR server software (basically DHCP for Cisco Cable
modem networks).  Just like a Ethermac on a NIC HFC MACs are unique MAC
address (they even have vendor codes just like ethermacs) and thus are the
perfect way to create a truly unique modem.  When the Help Desk tech had you
power cycle it was due to the fact that he cleared the host off the UBR, and
more than likely made a provisioning request to someone in their
Provisioning Dept to push a new tftp file (.cm file or a small binary that
configs your modem) to your modem. Since AT&T uses provisioning on a per
modem basis to prevent a "wide open" cable network in which anyone with a
DOCSIS modem could plug it in and work (believe me this is more common than
you might think) it wasn't until this request was filled manually by someone
that your modem was registered once again on the UBR.  Thus your service
began to work again once you pulled a new IP.  Now, if AT&T has your .cm
file set to (or you purchase a QOS which allows I should say) allow more
than one CPE address than you can have multiple routable IPs assigned to
host behind your modem. Thus you could have your Linksys with one IP and
another host with another Routable IP. (Dunno, if the Linksys will act
simply as a bridge and not try to NAT the IP or not ??). Anyway. This is
nothing new, all cable modem ISP's do the same thing. It appears that in
your case, once again human error was the problem.

thanks,
itallushrt

"Leonard Evens" <len@math.northwestern.edu> wrote in message
news:20020205.083753.1139901474.1418@math.northwestern.edu...
> I learned something new about how the new AT&T cable internet
> service works which may be of use to others.   It is a long sad tale, so
> bear with me, because you might find the punch line interesting.
>
> The whole thing started because I was foolish enough to change my TV
> cable package.  When that took effect, I lost my internet connection.
> After lots of time on the phone talking to the higher
> echelon techs, we discovered that long ago, perhaps even
> when Mediaone owned the cable company, someone entered the
> wrong number of connections in my house in their database    As a result,
> when I changed the TV package, their program suspended my internet
service.
>
> I normally use a Linksys dsl/cable router, but to try to get me connected
> again, the AT&T techs of course insisted that the cable modem be
> connected directly to a computer running Windows.   They also wanted
> me to power off/on the modem, which resulted in something like a half
> hour wait or longer before it managed to negotiate a connection with the
> AT&T network.
>
> In any case, eventually the modem made a connection and I got an IP
> address which was different from the one assigned earlier and
> which I had in fact been using since the Excite->AT&T
> switchover/disaster.   This was of course with the modem connected
> directly to the ethernet card in the computer.
>
> I then reconnected everything through the dsl/cable router, and strange
> things started happening.   I remained connected in the sense that my
> browsers (on two different machines using the router) saw the internet.
> But I couldn't connect to my office computer in which I had jury rigged
> the firewall so it had to know my home IP address.   The problem, I
> discovered was that my router kept using the old IP address which had
> been active until AT&T suspended my account, and nothing I could
> do could convince  it to use the new address.   I tried renewing the
> lease to the wan.  I tried switching to static addressing using the new
> address, which did work, but when I switched back to dynamic addressing
> it came back with the old address again.   I even powered off/on the
> router, but that also made no difference.
>
> Finally, I remembered from a previous newsgroup discussion that AT&T was
> using the Mac ID to verify its connection.   I also remembered that
> someone had noted that the Linksys allowed you to clone any Mac ID you
> wanted.   With a little effort, I found where the Linksys firmware had
> hidden that option---not where the manual said it would be---and cloned
> the hardware address of the ethernet card the cable modem had been
> connected to when the connection was made.   I then renewed the lease,
> and Voila!  I got the new IP address they had assigned me.   I then got
into
> my office computer by an alternate route and told it what it needed to
> know, and now everything is back to normal.
>
> I am still a bit confused about just what was going on with the Linksys
> router.   My conjecture is that AT&T never recovered the original IP
> address and in fact it is still assigned to the native Mac ID of the
> router.   So when their network saw the router it used that address.  On
> the other hand something else in their system knew what the new address
> was and when my office computer tried to access the old address to verify
> ssh, it wouldn't accept it.   If anyone has a better idea of what was
> going on,  I would be interested in hearing it.
>
> I presume that if I had turned off both router and modem, then when I
> turned them on again, a new connection might have been made using the
> native Mac ID of the router.   But I don't want to chance disconnecting
> the modem from their network if I don't have to, given how long it took
> to reestablish a connection.   I certainly hope I don't have to go
> through the whole thing again, including spending an hour or more on the
> phone the next time we have a power failure!
>
>
> --
> Leonard Evens      len@math.northwestern.edu      847-491-5537
> Dept. of Mathematics, Northwestern Univ., Evanston, IL 60208


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