[HFC] pooling modems in layer2

Rob Seastrom rs at seastrom.com
Wed Aug 13 01:32:41 UTC 2014

"Toney Mareo" <halflife4 at gmx.com> writes:

> Hello
> I think it's kind of an isp secret but I would be curious how do
> people distribute modems to pools before they would even reach the
> actual IP network so on layer2:
> http://dl.packetstormsecurity.net/papers/evaluation/docsis/Service_Distribution.jpg

Nobody does CMTRI anymore.  That illustration is over a decade and a
half old, which is part of what's confusing you.  The scheme there is
that they use a dialup modem for the upstream and a cablemodem for the

> For this I would like to get some clarification because I do not work in the telco industry.

If you're interested in how CMTRI works for historical reasons, the spec is here:


> As I can figure out of the docsis, cablelabs documents. The CMTS
> device is connected to the coax segments through fiber. Therefore
> one could say that the "modem facing" side is a fiber optic
> interface but it's not 1000 Base-FX, not a regular Ethernet over
> fiber. It sends signals through a broad range of frequencies.

It sends signals over RF (i.e. truly "broadband").  The RF happens to
be on a laser-lit fiber instead of a piece of coax (until it hits the
fiber node and gets turned into coax cable).  There are Ethernet MAC
addresses in there if you look at the right layer, but the DOCSIS data
rides as a "program" atop a J.83 single program transport stream on a
QAM64 or QAM256 modulated RF signal.  It's just like a digital TV
program and occupies the same frequency space - but 0x1FFE is the
well-known PID that means "DOCSIS data".

The upstream channels are comparatively low (under 80 MHz) and the
downstream channels are comparatively high (over 80 MHz to 800-1000
MHz depending on the system).  Splitting them out is accomplished with
bidirectional high and low pass filters called "diplexers".

> So what I would like to accomplish to provide a different pool of
> dhcp servers, which provides different config file, tod server,
> router, dns etc. infos to the modems but to do all this in Layer2.
> I don't have hands on experience with CMTS-es but I would think that
> they are able to pool clients by MACs and able to send eg 500
> clients to DHCP server1 and the other 1500 to DHCP server2 before
> they would even get an IP, so I talking of pure layer2 here!

There are multiple ways to approach this.  You need a consultant who
is well-versed in the care and feeding of DOCSIS edge networks to walk
through your options with you so that you don't find yourself in a
painful technical place.

> Let's say if the CMTS device does not support this, what are the
> other options for routing layer2 traffic coming out of the CMTS?

I don't recommend PPPoE.  :)

> If I would know more about the device I would say that put a
> linuxbox after it (on the ISP facing nic) and mark the packets going
> out with arptables/ebtables then send them out of different nics to
> different dhcp servers.
> Any suggestions are welcome.

You might start by sharing a high level overview of what it is that
you're trying to accomplish.  If it's simply sandboxing people who
haven't paid their bills, there are well-known ways to do that.  If
it's business services over DOCSIS, there are likewise ways to do


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