Internet-Draft                                          M. Lind (ed.)
   <draft-lind-v6ops-isp-scenarios-01.txt>                  J. Mulahusic
   Expires : April 2004                                      TeliaSonera
                                                                 D. Park
                                                    Samsung  Electronics
                                                               A. Baudot
                                                          France Telecom
                                                               P. Savola
                                                            October 2003
                Scenarios for Introducing IPv6 into ISP Networks
   Status of this Memo
      This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026.
      Internet-Drafts are working documents of the Internet Engineering
      Task Force (IETF), its areas, and its working groups.  Note that
      other groups may also distribute working documents as Internet-
      Internet-Drafts are draft documents valid for a maximum of six
      months and may be updated, replaced, or obsoleted by other documents
      at any time.  It is inappropriate to use Internet-Drafts as
      reference material or to cite them other than as "work in progress".
      The list of current Internet-Drafts can be accessed at
      The list of Internet-Draft Shadow Directories can be accessed at
      This document describes different scenarios for the introduction of
      IPv6 into an existing IPv4 ISP network without disrupting the IPv4
      service. During the IPv6 introduction can the network go through
      different stages. The first one is the initial stage of an IPv4-only
      infrastructure, and the final one corresponds to a whole dual-stack
      infrastructure enabling full coexistence of IPv4 and IPv6 traffic
      and services. In between, typical intermediate stages involving
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      coexistence mechanism are identified. The scenarios depicted in this
      document are describing the way to move along these different
      possible stages.
   Table of Contents
   1.    Introduction.................................................2
   2.    Scope of the document........................................3
   3.    Terminology used.............................................3
   4.    Brief description of a generic ISP network...................4
   5.    Scenarios....................................................6
         5.1  Assumptions.............................................6
         5.2  Customer requirements and ISP offerings.................7
         5.3  Stage 1 Scenarios: Launch...............................8
         5.4  Stage 2a Scenarios: Core................................9
         5.5  Stage 2b Scenarios: Access..............................9
         5.6  Stage 2a and 2b combination scenarios..................11
         5.7  Stage 3 scenarios: Complete............................12
         5.8  Impact on the "IT infrastructure"......................13
   6.    Transition Scenarios........................................14
   7.    Future Stages...............................................15
   8.    Example networks............................................15
         8.1  Example 1..............................................16
         8.2  Example 2..............................................17
         8.3  Example 3..............................................17
         8.4  Example 4..............................................18
   9.    Security Considerations.....................................18
   1. Introduction
      An ISP offering an IPv4 service will find that there are different
      ways to add IPv6 to this service.  During the introduction of IPv6
      the network will go through different stages of IPv6 maturity.  In
      addition to this there has to be a transition between these stages
      to make them feasible to implement.  The main goal of this document
      is to provide possible scenarios to the ISP when introducing IPv6
      connectivity in the existing ISP IPv4 legacy network.
      In this document different transition scenarios and situations
      during the introduction of IPv6 are covered in a broader perspective
      and deals only with a generic view of how an ISP network is built.
      This should be seen as the starting point for further documentation
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      in a companion document of how the introduction of IPv6 can be done
      in an ISP network.
   2. Scope of the document
      The scope of the document is to describe different cases for the
      introduction of an IPv6 service in a generic IPv4 ISP network. This
      means that the document will be limited to services that include
      both IPv6 and IPv4 and will not cover issues surrounding an IPv6
      only service. Therefore, the ISP network should be able to carry
      IPv4 and IPv6 traffic without any distinction related to the version
      of the protocol.
      The different building blocks that will be considered are the
      customer network, the access networks, the core network and exchange
      The network can be at a different stage relating to, either how far
      it has adopted IPv6, or to how likely it may be upgraded to IPv6. We
      will consider these stages, as well as the transition scenarios
      between the different stages.
      It is outside the scope of this document to describe different types
      of access or network technologies. It is also outside of the scope
      to propose different solutions. Solutions will be covered in a
      separate document.
   3. Terminology used
      This section defines and clarifies the terminology used in this
      "CPE"         : Customer Premise Equipment
      "PE"          : Provider Edge equipment
      "Access"      : This is the part of the network which is used by a
                      customer when connecting to an ISP network. It
                      includes the CPEs, the last hop links and the parts
                      of the PE interfacing to the last hop links.
      "Core"        : This is the rest of the ISP network infrastructure.
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                      It includes the parts of the PE interfacing to the
                      core backbone, the core routers of the ISP and the
                      border routers used in order to exchange routing
                      information with other ISPs (or other administrative
      "IT infrastructure" :
                      This is the part of the ISP network which hosts the
                      services required for the correct operation of the
                      ISP network. It usually includes DNS servers,Radius
                      servers, monitoring and configuration
      "Dual network": A network which supports natively both IPv4 and
   4. Brief description of a generic ISP network
      A generic ISP network topology can be divided into two main parts;
      the core network and the access networks connecting the customers.
      The core network or the backbone is the part of the network that
      interconnects the different access networks and provides transport
      to the rest of the Internet via exchange points or other means. The
      core network can be built on different technologies but in this
      document the only interest is whether it is capable of carrying IPv6
      traffic natively or not. Since there is no clear definition of core,
      it is defined in this document as being all routers that are a part
      of the same routed domain in the transport network. This means that
      all routers up to the PE router are a part of the core. The PE
      router can also be partially part of the core if it exchanges
      routing information and transports traffic to and from the core.
      The access networks provide connectivity to enterprise and private
      customers. Other ISPs might as well be customers and connected to
      the ISP's access network. As with the core the absence or presence
      of native IPv6 capability is the only thing of real interest in the
      access network technology.
      It is noticeable that, in some cases (e.g. European legacy
      operators), a given access network may have to be shared between
      different ISPs. According to the type of the access network used
      (e.g. involving only layer 2 devices, or involving non-IP
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      technology), this constraint result into architectural
      considerations that may be relevant in the analysis document.
      "IT infrastructure" building blocks refer to the basic main
      functions needed for a regular backbone operation. This building
      block is dealing with: network management, customers' authentication
      and accounting, address assignment and naming. It represents the
      minimum functions needed to provide a customer service, referring to
      both network infrastructure operation, and administrative management
      of customers.
      It doesn't matter if these customer networks have a single node or a
      large routed network. What is of interest is if routing information
      is exchanged or not since it will affect the ISP's network. The
      existence of customer placed equipment will also affect how a
      service can be delivered. In addition to the ISP's actual network
      components there are a lot of support and backend systems that have
      to be considered.
           ------------    ---------
          |    IT      |  |         |
          |infrastruct.|--|  CORE   |
          |            |  |         |\
           ------------    ---------  \
              .             / | \      \
              .            /  |  \      \_Peering( Direct & IX )
              .           /   |   \
              .          /    |    \
              .         /     |     \
           -------     /   -------   \     -------
          |       |   /   |       |   \   |       |
          |Access1|--/    |Access2|    \--|Access3|
          |       |       |       |       |       |
           -------         -------         -------
              |               |               | ISP Network
              |               |               | Customer Networks
          +--------+      +--------+      +--------+
          |        |      |        |      |        |
          |Customer|      |Customer|      |Customer|
          |        |      |        |      |        |
          +--------+      +--------+      +--------+
         Figure 1: ISP network topology
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   5. Scenarios
      The scenarios section describes different stages an ISP might
      consider when introducing IPv6 connectivity in the existing IPv4
      network and the different scenarios that might occur in the
      respective stages.
      The stages here are snapshots of an ISP's network with respect to
      IPv6 maturity.  Since an ISP's network is constantly evolving, a
      stage is a measure of how far an ISP has come in turn of
      implementing necessary functionality to offer IPv6 to the customers.
      It is possible to freely transition between different stages.
      However, a network segment can only be in one stage at a time but an
      ISP network as a whole can be in different stages.  There are
      different transition paths between the first and final stage.  The
      transition between two stages does not have to be immediate but can
      occur gradually.
      Each stage has different IPv6 properties.  An ISP can therefore,
      based on the requirements it has, decide into which stage it will
      transform its network.
   5.1 Assumptions
      The stages are derived from the generic description of an ISP
      network in section 3.  A combination of different building blocks
      that constitute an ISP environment will lead to a number of
      scenarios, which an ISP can choose from.  The scenarios of most
      relevance to this document are considered to be the ones that in the
      most efficient and feasible way maximize the ability for an ISP to
      offer IPv6 to its customers.
      The most predominant case today is considered to be an operator with
      a core and access IPv4 network delivering IPv4 service to a customer
      that is running IPv4 as well.  At some point in the future, it is
      expected that the customer will want to have an IPv6 service, in
      addition to the already existing IPv4 service. This IPv6 service may
      be offered by the same ISP, or by a different one. Anyway the
      challenge for the ISP is to deliver the requested IPv6 service over
      the existing infrastructure and at the same time keep the IPv4
      service intact.
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   5.2 Customer requirements and ISP offerings
      Looking at the scenarios from the end customer's perspective there
      might be a demand for three different services; the customer might
      demand IPv4 service, IPv6 service or both services.  This can lead
      to two scenarios depending on the stage the ISP's network is in.
      If an ISP only offers IPv4 or IPv6 service and a customer wants to
      connect a device or network that only supports one service and if
      that service is not offered, it will lead to a dead-end.  If the
      customer or ISP instead connects a dual stack device it is possible
      to circumvent the lack of the missing service in the access network
      by using some kind of coexistence mechanism.  This scenario will
      only be considered in the perspective of the ISP offering a
      mechanism to bridge the access and reach the IPv6 core.
      In the case where the customer connects a single stack device to a
      corresponding single stack access network or when the customer
      connects a single stack device to a dual stack access network is
      covered by the all over dual stack case. Therefore, neither of these
      cases need further be explored separately in this document but can
      be seen as a part of a full dual stack case.
      After eliminating a number of cases explained above, there are four
      stages that are most probable and where an ISP will find its network
      in.  Which stage a network is in; depends on whether or not some
      part of the network previously has been upgraded to support IPv6 or
      if it is easier to enable IPv6 in one part or another.  For
      instance, routers in the core might have IPv6 support or might be
      easily upgradeable, while the hardware in the access might have to
      be completely replaced in order to handle IPv6 traffic.
      Note that in every stage except Stage 1, the ISP can offer both IPv4
      and IPv6 services to the customer.
      The four most probable stages are:
            o Stage 1      Launch
            o Stage 2a     Core
            o Stage 2b     Access
            o Stage 3      Complete
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      Generally the ISP is able to upgrade current IPv4 network to
      IPv4/IPv6 dual network via Stage 2b but the IPv6 service can also be
      implemented at a small cost with simple tunnel mechanisms on the
      existing system.  When designing a new network Stage 3 might be the
      first and last step since there is no legacy concerns. Absence of
      IPv6 capability in the network equipment can still be a limiting
      factor nevertheless.
   5.3 Stage 1 Scenarios: Launch
      The first stage is an IPv4 only ISP with an IPv4 customer. This is
      the most common case today and has to be the starting point for the
      introduction of IPv6.  From this stage, an ISP can move (transition)
      to any other stage with the goal to offer IPv6 to its customer.
      The scenario and the immediate first step is to get a prefix
      allocation (typically a /32) from the appropriate RIR according to
      allocation process. For the IPv6 migration scenarios described in
      this document, an ISP has to be able to exchange IPv6 traffic, e.g.
      by connecting to an exchange, through a direct peering/transit or a
      tunnel, prior to introducing customers in Stage2 and Stage 3.
      Customer    Access      Core     Exchange
      +------+   +------+   +------+   +------+
      |      |   |      |   |      |   | IPv4 |
      | IPv4 |---| IPv4 |---| IPv4 |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
               Figure 2. IPv4 network
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   5.4 Stage 2a Scenarios: Core
      Stage 2a is an ISP with access networks that are IPv4 only and a
      core that is IPv4 and IPv6.  In particular, the ISP considers it
      possible to make the core IPv6 capable either through software or
      hardware upgrades. In this stage the customer should have support
      for both IPv4 and IPv6 and use a tunneling mechanism to be able to
      run the IPv6 service. To offer the IPv6 service, the ISP also has to
      exchange IPv6 traffic with other ISPs e.g. by connecting to an IPv6
      exchange point. In particular, An ISP has to provide IPv6
      connectivity through its IPv4 access networks.
      An ISP can consider two kinds of scenarios such as automatic tunnels
      (e.g. provided by the 6to4 mechanism) and configured tunnels to
      bring IPv6 connectivity on top of an IPv4 only service.  Both
      methods have advantages and limitations which are out of scope in
      this document and will be covered in the analysis document. The
      existence of NATs and firewalls in the path is also to be
      Customer    Access      Core     Exchange
      +------+   +------+   +------+   +------+
      |      |   |      |   |      |   | IPv4 |
      | Dual |---| IPv4 |---| Dual |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
                   IPv6         <---IPv6--->
        Tunnel via access network
             Figure 3. Upgraded core
   5.5 Stage 2b Scenarios: Access
      Stage 2b is an ISP with a core network that is IPv4 and an access
      network that is IPv4 and IPv6. Since the service to the customer is
      native IPv6 there is no requirement for the customer to support both
      IPv4 and IPv6. This is the biggest difference in comparison to the
      previous stage.  The need to exchange IPv6 traffic or similar still
      exists but might be more complicated than in the previous case since
      the core isn't IPv6 enabled. After completing stage 2b the original
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      IPv4 core still is unchanged. This doesn't imply that there is no
      IPv6 core just that the IPv6 core is an overlay to or partially
      separated from the IPv4 core.
      Like in section 5.4 tunnels is a possible scenario and can be used
      for IPv6 connectivity over the IPv4 network parts. Other forms of
      transport over for example an MPLS enabled core are also possible
      Generally, the ISP will continue providing IPv4 connectivity; in
      many cases private addresses and NATs will continue to be used.
      Access networks should make use of a mechanism to delegate a global
      IPv6 address prefix from the ISP to the customer.
      Customer    Access      Core     Exchange
      +------+   +------+   +------+   +------+
      |      |   |      |   |      |   | IPv4 |
      | Dual |---| Dual |---| IPv4 |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
       <---IPv6--->          IPv6
                   Tunnel via core network
               Figure 4. Upgraded access
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   5.6 Stage 2a and 2b combination scenarios
      Some ISPs may use different access technologies of varying IPv6
      maturity. This may results in a combination of the former stages.
      The case depicted in the figure 5 below, has no impact on stage 2a
      since it results in interconnection a dual access network to a dual
      core network.
      Customer A  Access 1
      +------+   +------+
      |      |   |      |
      | Dual |---| Dual |---+
      |      |   |      |   |
      +------+   +------+   |
      Customer B  Access 2   \ Core    Exchange
      +------+   +------+   +-+----+   +------+
      |      |   |      |   |      |   | IPv4 |
      | Dual |---| IPv4 |---| Dual |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
                   IPv6         <---IPv6--->
        Tunnel via access network
               Figure 5. Upgraded core with multiple access
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      The case depicted in the figure 6 below, results in tunnel chaining,
      in order to keep independent access and core upgrade that may
      happened according to totally different timeframe.
         <--IPv4--> | <------IPv4------->
         +=========+ +==================+
            IPv6             IPv6
         +=========+ +==================+
        Tunnel via       Tunnel via
      access network     Core network
      Customer    Access
      +------+   +------+
      |      |   |      |
      | Dual |---| IPv4 |---+
      |      |   |      |   |
      +------+   +------+   |
      Customer    Access      | Core   Exchange
      +------+   +------+   +-+----+   +------+
      |      |   |      |   |      |   | IPv4 |
      | Dual |---| Dual |---| IPv4 |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
   <---IPv6--->          IPv6
                   Tunnel via access network
               Figure 6. Upgraded access with upgrade core
   5.7 Stage 3 scenarios: Complete
      Stage 3 can be said to be the final step in introducing IPv6, at
      least in the scope of this document.  This is an all over IPv6
      service with native support for IPv6 and IPv4 in both core and
      access networks.  This stage is identical to the previous stage in
      the customer's perspective since the access network hasn't changed.
      The requirement for exchanging IPv6 traffic is identical to stage 2.
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      Customer    Access      Core     Exchange
      +------+   +------+   +------+   +------+
      |      |   |      |   |      |   | IPv4 |
      | Dual |---| Dual |---| Dual |---|   +  |
      |      |   |      |   |      |   | IPv6 |
      +------+   +------+   +------+   +------+
           Figure 7. Completely upgraded network
   5.8 Impact on the "IT infrastructure"
      The different stages above are dealing with fundamental issues such
      as IPv6 connectivity and IPv6 traffic forwarding.
      Some other background tasks must be realized in parallel to complete
      the new IPv6 service. The main tasks identified are:
      - Customer authentication and accounting
      - Address assignment
      - Network management
      - Naming service
      Customer authentication and accounting and address assignment are
      relevant to the access network, and address assignment may have an
      impact on the core network.
      Network management is relevant to both access and core networks.
      Naming service intends to address minimum DNS facilities an ISP may
      have to provide.
      From a general point of view these functions may be realized based
      on an IPv4 transport layer, an IPv6 transport layer, or both.
      A service, such as a web server, advertised by an IPv6 address must
      be reachable from any IPv6 node.
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   6. Transition Scenarios
      Given the different stages it is clear that the ISP has to be able
      to transition from one stage to another.  The initial stage, in this
      document, is the IPv4 only service and network.  The end stage is
      the dual IPv4/IPv6 service and network.  As mentioned in the scope,
      this document does not cover the IPv6 only service and network and
      its implications on IPv4 customers.  This has nothing to do with the
      usability of an IPv6 only service.
      The transition starts out with the IPv4 ISP and then moves to one of
      three choices.  These choices are the different transition
      scenarios. One way would be to upgrade the core first which leads to
      stage 2a. Another way to go could be to upgrade the access network
      which leads to stage 2b.  The final possibility is to introduce IPv6
      in the whole network at once which would lead to stage 3.
      The choice is dependent on many different issues.  For example it
      might not be possible to upgrade the core or the access network
      without large investments in new equipment which could lead to any
      of the two first choices.  In another case it might be easier to
      take the direct step to a complete IPv6/IPv4 network due to routing
      protocol issues or similar.
      If a partially upgraded network (stage 2a or 2b) was chosen as the
      first step, a second step remains before the network is all over
      native IPv6/IPv4.  This is the transition to an all over dual stack
      network.  This step is perhaps not necessary for stage 2b with an
      already native IPv6 service to the customer but might still occur
      when the timing is right.  For stage 2a it is more obvious that a
      transition to a dual stack network is necessary since it has to be
      done to offer a native IPv6 service.
      As most of the ISPs keep evolving continuously their core IPv4
      networks (new firmware versions in the routers, new routers), they
      will be able to get them IPv6 ready, without additional investment,
      except the staff training. It may be a slower transition path, but
      useful since it allows an IPv6 introduction without any actual
      customer demand. This will probably be better than making everything
      at the last minute with a higher investment.
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   7. Future Stages
      After a while the ISP might want to transition to a service that is
      IPv6 only, at least in certain parts of the network.  This
      transition creates a lot of new cases in which to factor in how to
      maintain the IPv4 service.  Providing an IPv6 only service is not
      much different than the dual IPv4/IPv6 service described in stage 3
      except from the need to phase out the IPv4 service.  The delivery of
      IPv4 services over an IPv6 network and the phase out is left for a
      future document.
   8. Example networks
      In this section, a number of different network examples are
      presented. They are only example networks and will not necessary
      match to any existing networks. Nevertheless, the examples will
      hopefully be useful even in the cases when they do not match the
      target networks. The purpose of the example networks is to exemplify
      the applicability of the transition mechanisms described in this
      document on a number of different example networks with different
      The example network layout will be the same in all network examples.
      The networks examples are to be seen as a specific representation of
      the generic network with a limited number of network devices. An
      arbitrary number (in this case 7) of routers have been selected to
      represent the network examples. However, since the network examples
      follow the implementation strategies recommended for the generic
      network scenario, it should be possible to scale the example to fit
      a network with an arbitrary number, e.g. several hundreds or
      thousands, of routers.
      The routers in the example are interconnected with each other as
      well as with another ISP. The connection to another ISP can either
      be a direct connection or through an exchange point. In addition to
      these connections, there are also a number of access networks
      connected to the routers. Access networks are normally connected to
      the core via access routers, but can in some cases be directly
      connected to the core routers. As described earlier in the generic
      network scenarios, the access networks are used to connect the
      customers. Access networks can, for example, be xDSL or cable
      network equipment.
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                     ISP1 | ISP2
                +------+  |  +------+
                |      |  |  |      |
                |      |  |  |      |
                +------+  |  +------+
                /      \  +-----------------------
               /        \
              /          \
          +------+    +------+
          |      |    |      |
          |      |    |      |
          +------+    +------+\
              |           |    \             | Exchange point
          +------+    +------+  \  +------+  |  +------+
          |      |    |      |   \_|      |  |  |      |--
          |      |    |      |     |      |  |  |      |--
          +------+   /+------+     +------+  |  +------+
              |     /     |                  |
          +-------+/  +-------+              |
          |       |   |       |
          |Access1|   |Access2|
          |       |   |       |
          +-------+   +-------+
            |||||       |||||  ISP Network
              |           |    Customer Networks
          +--------+  +--------+
          |        |  |        |
          |Customer|  |Customer|
          |        |  |        |
          +--------+  +--------+
                Figure 2: ISP network example
   8.1 Example 1
      In example 1 a network built according to the example topology is
      present with a native IPv4 core, the routers. The core is running
      IS-IS and IBGP as routing protocol for internal and external routes
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      respectively. In the connection to ISP2 and the exchange point MBGP
      is used to exchange routes. Multicast is present and is using PIM-SM
      routing. QoS is present and is using DiffServ.
      Access 1 is xDSL, connected to the core through an access router.
      The xDSL equipment, except for the access router, is considered to
      be layer 2 only, e.g. Ethernet or ATM. IPv4 addresses are
      dynamically assigned to the customer using DHCP. No routing
      information is exchanged with the customer. Access control and
      traceability is done in the access router. Customers are separated
      in VLANs or separate ATM PVCs up to the access router.
      Access 2 is Fiber to the building/home connected directly to the
      core router. The FTTB/H is considered to be layer 3 aware and
      performs access control and traceability through its layer 3
      awareness. IPv4 addresses are dynamically assigned to the customers
      using DHCP. No routing information is exchanged with the customer.
   8.2 Example 2
      In example 2 the core is running IPv4 with MPLS. Routing protocols
      used are OSPF and IBGP for internal and external routes. In the
      connection to ISP2 and the exchange point BGP is used to exchange
      routes. Multicast and QoS are not present.
      Access 1 is a fixed line access, e.g. fiber, connected directly to
      the core. CPE is present at the customer and routing information is
      in some cases exchanged otherwise static routing is used. Access 1
      can also be connected to BGP/MPLS-VPN running in the core.
      Access 2 is xDSL connected directly to the core router. The xDSL is
      layer 3 aware. Addresses are dynamically assigned using DHCP. Access
      control is achieved on the physical layer and traceability is
      achieved using DHCP snooping. No routing information is exchanged
      with the customer.
   8.3 Example 3
      A transit provider offers IP connectivity to other providers, but
      not to end users or enterprises. IS-IS and IBGP is used internally
      and BGP externally. Its accesses connect Tier-2 provider cores. No
      multicast or QoS is used.
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   8.4 Example 4
      Yet another example, if needed. To be done.
   9. Security Considerations
      This document describes different scenarios for the introduction of
      IPv6 in an IPv4 ISP network.  Solutions are described in other
      documents hence this document has no security considerations.
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   Full Copyright Statement
      Copyright (C) The Internet Society (2003). All Rights Reserved.
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      or assist in its implementation may be prepared, copied, published
      and distributed, in whole or in part, without restriction of any
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      kind, provided that the above copyright notice and this paragraph
      are included on all such copies and derivative works. However, this
      document itself may not be modified in any way, such as by removing
      the copyright notice or references to the Internet Society or other
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      The limited permissions granted above are perpetual and will not be
      revoked by the Internet Society or its successors or assignees.
      This document and the information contained herein is provided on an
      This draft is a joint effort of the ISP design team. The team
      consists of Jordi Palet, Suresh Satapati, Myung-Ki Shin, Vladimir
      Ksinant, Cleve Mickles, Marc Blanchet, Soohong Daniel Park, Alain
      Baudot and Mikael Lind
      This draft has also greatly benefited from input by Margaret
   Authors' Addresses
      Mikael Lind
      Vitsandsgatan 9B
      SE-12386 Farsta, Sweden
      Jasminko Mulahusic
      Vitsandsgatan 9B
      SE-12386 Farsta, Sweden
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      Soohong Daniel Park
      Mobile Platform Laboratory, SAMSUNG Electronics.
      416, Maetan-3dong, Paldal-Gu,
      Suwon, Gyeonggi-do, Korea
      Alain Baudot
      France Telecom R&D
      42, rue des coutures
      14066 Caen - FRANCE
      Pekka Savola
      Espoo, Finland
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