[HFC] pooling modems in layer2

charles at thefnf.org charles at thefnf.org
Tue Aug 12 17:51:23 UTC 2014

On 2014-08-12 09:23, Toney Mareo 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
> 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.

Sounds about right to me.

> 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? Do you have a bunch of cable modems and a CMTS? If so, does the 
documentation not cover this? Or are you trying to hack your cable 
modem/cable provider?

> 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!
> 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?

Um. Probably via RADIUS and via VLAN assignment?

  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.

Most likely they just use VLANs. This rack of CMTS gear is on port 22 of 
the agg switch, vlan 2 and ip helper is set for vlan 2 to the desired 
dhcp server (which is most likely an HA floating IP if not a full blown 
VIP etc).

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