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Re: Fwd: Re: DOCSIS 3.0 & PPPoE/L2TP compatibility
From: Scott Helms <khelms () ispalliance net>
Date: Thu, 02 Aug 2012 12:59:17 -0400


That's only true if you want to truly implement transparent LAN services over DOCSIS. Separating the CPE data flow works with any DOCSIS 1.0 or better modem since all of the tricky parts are in the CMTS. We took a municipal cable network through 3 different CMTSs (3Com and then a Terayon Be2k and finally an Arris C4). The first two did PPPoE hand off to a Redback to act as the LNS and when they wanted to move up to a bigger CMTS the city choose Arris and PPPoE stopped being an option. In short, replacing the modems isn't a requirement unless the modem has to pass up the TLS data, which isn't the case in an open access ISP scenario.

On 8/1/2012 3:40 PM, Brian Mengel wrote:
One thing to be mindful of is that BSoD support may not be prevelant
in the installed modem base of your MSO.  Replacing those modems would
be costly for someone.

On Wed, Aug 1, 2012 at 10:17 AM, iptech <iptech () northrock bm> wrote:
Hi Scott,

Thanks for the feedback,

yes this is how I understand it also, however I find it strange that the
Cisco platform designated as the future LNS will not accommodate the DOCSIS
3.0requirements - not much collaboration. There is no roadmap for
introcducing PPTP on the ASR1K that I can see, so i will not be holding out
for one.

I am veering towards using a L2 pw solution, but would be interested to hear
what you have used in-house yourself to accommodate this change, care to


On 7/31/2012 5:46 PM, Scott Helms wrote:

I've actually run into this specific problem and the issue your running
into is that at no time was PPPoE part of the DOCSIS specification.  It
was supported on several CMTSs  because the Cisco UBR shares much of its
OS with more mainline Cisco routers which support L2TP and a host of
other non-DOCSIS related protocols.  It was also widely supported on
some of the earliest CMTSs which were bridges instead of routers (then
you needed a separate box to be the LNS).  The real problem isn't a
change in DOCSIS version but that they choose a platform that doesn't
share a code base with a general purpose router. This could have been
happenstance or by design, but I can tell you your chances of getting
PPPoE to work at all on that platform (even for the cable operator) are
not high because the box will not operate as a bridge and there is no
(AFAIK) way to relay the PPP discover packets.

The D3 Arris is either a C4 or a C4C:

On 7/30/2012 8:33 AM, iptech wrote:

We are a small ISP and have a setup in place with the local cable
company for terminating their users via L2TP for Internet access.
However they have just announced to us that they are moving to a
DOCSIS 3.0 compliant setup, and this standard no longer supports PPPoE
via L2TP, and can now only offer PPTP for terminating with us.

We have already begun replacing our Cisco 7206VXR LNS devices with
Cisco ASR 1Ks and as you will be aware the older 7206 can do both L2TP
and PPTP, whereas the ASR1k can do only L2TP. I do not have any
experience in the cable arena, but from what I have read in the DOCSIS
standards, each version has maintained backwards compatibility,
therefore I am very surprised our CableCo has claimed they cannot do
PPPoE/L2TP anymore.

The CMTS they are currently using is a Cisco, and now they are moving
to a new ARRIS CMTS. I have not been able to find any information on
this device and what it can do or not. With the ASR1K marked as the
natural upgrade path for LNS functions, therefore I cannot believe
that it is not fully compatible with DOCSIS 3.0.

 From what I can tell the only way to accommodate the new CMTS PPTP
connections will be to terminate them on the legacy 7206VXR, which at
the end of the day is a backwards step. I would greatly appreciate if
anyone can give me any pointers and/or suggestions on this matter, so
I can understand it and move it forward.

FYI: The driver for the CMTS upgrades is to offer higher bandwidth
access speeds 15mb-20mb.

Thank you.

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

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