[HFC] pooling modems in layer2

Scott Helms khelms at zcorum.com
Tue Aug 12 18:11:54 UTC 2014

Scott Helms
Vice President of Technology
(678) 507-5000

On Tue, Aug 12, 2014 at 10:23 AM, Toney Mareo <halflife4 at gmx.com> wrote:

> 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

Certainly not secret, DOCSIS is a very well documented protocol with most
of the information being publicly available.

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

While fiber is commonly used in cable plants as part of a HFC network its
completely transparent from a protocol standpoint the entire communication
is over RF.  D3 and older uses QAM modulation and the downstream runs over
"normal" 6 MHz channels which are the same as TV channels.

> 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.

Why?  The operator is the only one who can tell the CMTS which DHCP
server(s) to send traffic to and modern CMTSs do that as an IP relay and
passes its IP address as the GIADDR.

> 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!

Not exactly, first in nearly all cases the DHCP communication is an IP
unicast rather than a layer 2 broadcast.  Second, the way that the DHCP
server is selected is normally based on the type of device so that modems
get a specific GIADDR, CPE (PCs, routers behind modems, etc) get another
one, and often the EMTA gets a third.  It might be possible to do that off
a count of devices, but if so it will be more of a load balancing scenario
rather than these specific 500 CMs get this DHCP server.  It is possible to
do open access in a DOCSIS system, but its very difficult and involves
creating filters in both the CMTS and CM configurations.

> 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? 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.

It doesn't really work that way, but the closest thing is a "soft" tunnel
that gets used for things like transparent LAN services, carrier WiFi, and
a few other use cases.


> Any suggestions are welcome.

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