ISP IPv6 Deployment Scenarios in Broadband Access Networks

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RFC 4779          ISP IPv6 Deployment Scenarios in BB       January 2007


6.2.  Deploying IPv6 in IPv4 DSL Networks

   There are three main design approaches to providing IPv4 connectivity
   over a DSL infrastructure:

   1.  Point-to-Point Model: Each subscriber connects to the DSLAM over
       a twisted pair and is provided with a unique PVC that links it to
       the service provider.  The PVCs can be terminated at the BRAS or
       at the Edge Router.  This type of design is not very scalable if
       the PVCs are not terminated as close as possible to the DSLAM (at
       the BRAS).  In this case, a large number of Layer 2 circuits has
       to be maintained over a significant portion of the network.  The
       Layer 2 domains can be terminated at the ER in three ways:

       A.  In a common bridge group with a virtual interface that routes
           traffic out.

       B.  By enabling a Routed Bridged Encapsulation feature, all users
           could be part of the same subnet.  This is the most common
           deployment approach of IPv4 over DSL but it might not be the
           best choice in IPv6 where address availability is not an
           issue.

       C.  By terminating the PVC at Layer 3, each PVC has its own
           prefix.  This is the approach that seems more suitable for
           IPv6 and is presented in Section 6.2.1.

           None of these ways requires that the CPE (DSL modem) be
           upgraded.

   2.  PPP Terminated Aggregation (PTA) Model: PPP sessions are opened
       between each subscriber and the BRAS.  The BRAS terminates the
       PPP sessions and provides Layer 3 connectivity between the
       subscriber and the ISP.  This model is presented in Section
       6.2.2.

   3.  Layer 2 Tunneling Protocol (L2TP) Access Aggregation (LAA) Model:
       PPP sessions are opened between each subscriber and the ISP Edge
       Router.  The BRAS tunnels the subscriber PPP sessions to the ISP
       by encapsulating them into L2TPv2 [RFC2661] tunnels.  This model
       is presented in Section 6.2.3.

   In aggregation models, the BRAS terminates the subscriber PVCs and
   aggregates their connections before providing access to the ISP.

   In order to maintain the deployment concepts and business models
   proven and used with existing revenue generating IPv4 services, the
   IPv6 deployment will match the IPv4 one.  This approach is presented



Asadullah, et al.            Informational                     [Page 28]