[173945] in North American Network Operators' Group
Re: [HFC] pooling modems in layer2
daemon@ATHENA.MIT.EDU (Toney Mareo)
Thu Aug 14 08:49:08 2014
X-Original-To: nanog@nanog.org
From: "Toney Mareo" <halflife4@gmx.com>
To: "Scott Helms" <khelms@zcorum.com>
Date: Thu, 14 Aug 2014 14:48:58 +0200
In-Reply-To: <CAMrdfRw=hFvxqeOA-zBPqvA7_aZzBfv2xSCgzQe88J0+nZYA3A@mail.gmail.com>
Cc: NANOG <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
Hello
=C2=A0
Thanks for the responses, I think it clarified a lot and I already started=
reading this CM-SP-L2VPN-I13-140403.pdf documentation.
What I need here is that existing clients are sent through ISP1 currently =
and I would like to add ISP2 for future clients without interfering anythin=
g with the current operations. Then later on move the old clients over to I=
SP2 as well.
=C2=A0
As I see it, this can only be done on the CMTS device not after it unless =
it's possible to relay packets from the cable side with their original HFC =
macs through the CMTS.
=C2=A0
Yes indeed I do not want to setup failover or balance DHCP servers, but I =
want to move every new subscriber to a different pool which gets directed t=
o a different DHCP server which then finally able to provide the modems wit=
h ips and other settings to be able to go out on ISP2.
=C2=A0
=C2=A0
On Tue, Aug 12, 2014 at 10:23 AM, Toney Mareo <halflife4@gmx.com> wrote:He=
llo
I think it's kind of an isp secret but I would be curious how do people di=
stribute modems to pools before they would even reach the actual IP network=
so on layer2:
http://dl.packetstormsecurity.net/papers/evaluation/docsis/Service_Distrib=
ution.jpg[http://dl.packetstormsecurity.net/papers/evaluation/docsis/Servic=
e_Distribution.jpg]
=C2=A0
Certainly not secret, DOCSIS is a very well documented protocol with most =
of the information being publicly available.
=C2=A0
For this I would like to get some clarification because I do not work in t=
he 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 i=
t's not 1000 Base-FX, not a regular Ethernet over fiber. It sends signals t=
hrough a broad range of frequencies.
=C2=A0
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. =C2=A0D3 and older uses QAM modulation and the downstream runs =
over "normal" 6 MHz channels which are the same as TV channels.
=C2=A0
=C2=A0
So what I would like to accomplish to provide a different pool of dhcp ser=
vers, which provides different config file, tod server, router, dns etc. in=
fos to the modems but to do all this in Layer2.
=C2=A0
Why? =C2=A0The operator is the only one who can tell the CMTS which DHCP s=
erver(s) to send traffic to and modern CMTSs do that as an IP relay and pas=
ses its IP address as the GIADDR.
=C2=A0
Because I advise the operator, you would think they are expert on the CMTS=
? Think again, I'm not an expert either but at least I learning.
=C2=A0
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 se=
rver1 and the other 1500 to DHCP server2 before they would even get an IP, =
so I talking of pure layer2 here!
=C2=A0
Not exactly, first in nearly all cases the DHCP communication is an IP uni=
cast rather than a layer 2 broadcast. =C2=A0Second, the way that the DHCP s=
erver is selected is normally based on the type of device so that modems ge=
t a specific GIADDR, CPE (PCs, routers behind modems, etc) get another one,=
and often the EMTA gets a third. =C2=A0It 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. =C2=A0It is possi=
ble to do open access in a DOCSIS system, but its very difficult and involv=
es creating filters in both the CMTS and CM configurations.
=C2=A0
Let's say if the CMTS device does not support this, what are the other opt=
ions for routing layer2 traffic coming out of the CMTS? If I would know mor=
e about the device I would say that put a linuxbox after it (on the ISP fac=
ing nic) and mark the packets going out with arptables/ebtables then send t=
hem out of different nics to different dhcp servers.
=C2=A0
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.=C2=A0
=C2=A0
http://www.cablelabs.com/wp-content/uploads/specdocs/CM-SP-L2VPN-I09-10061=
1.pdf[http://www.cablelabs.com/wp-content/uploads/specdocs/CM-SP-L2VPN-I09-=
100611.pdf]
=C2=A0
Any suggestions are welcome.
