DOCSIS 3.0 & PPPoE/L2TP compatibility

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:

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 share?


We ended up using something like this to separate out the traffic at layer 2 for each ISP:

Look at section 5.1.2 Multiple ISP L2VPNs

Basically the modems get DHCP & their config from the cable operator but the CPEs get DHCP (and IP connectivity) from the individual ISPs.

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.


     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.