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Network Working Group M. Lind Request for Comments: 4029 TeliaSonera Category: Informational V. Ksinant

                                                 Thales Communications
                                                               S. Park
                                                   SAMSUNG Electronics
                                                             A. Baudot
                                                        France Telecom
                                                             P. Savola
                                                            March 2005
   Scenarios and Analysis for Introducing IPv6 into ISP Networks

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).


 This document describes different scenarios for the introduction of
 IPv6 into an ISP's existing IPv4 network without disrupting the IPv4
 service.  The scenarios for introducing IPv6 are analyzed, and the
 relevance of already defined transition mechanisms are evaluated.
 Known challenges are also identified.

Table of Contents

 1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
      1.1.  Goal and Scope of the Document. . . . . . . . . . . . .  2
 2.   Brief Description of a Generic ISP Network. . . . . . . . . .  3
 3.   Transition Scenarios. . . . . . . . . . . . . . . . . . . . .  4
      3.1.  Identification of Stages and Scenarios. . . . . . . . .  4
      3.2.  Stages. . . . . . . . . . . . . . . . . . . . . . . . .  5
            3.2.1.  Stage 1 Scenarios: Launch . . . . . . . . . . .  5
            3.2.2.  Stage 2a Scenarios: Backbone. . . . . . . . . .  6
            3.2.3.  Stage 2b Scenarios: Customer Connection . . . .  6
            3.2.4.  Stage 3 Scenarios: Complete . . . . . . . . . .  7
            3.2.5.  Stages 2a and 3: Combination Scenarios. . . . .  7
      3.3.  Transition Scenarios. . . . . . . . . . . . . . . . . .  7
      3.4.  Actions Needed When Deploying IPv6 in an ISP's Network.  8

Lind, et al. Informational [Page 1] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 4.   Backbone Transition Actions . . . . . . . . . . . . . . . . .  9
      4.1.  Steps in the Transition of Backbone Networks. . . . . .  9
            4.1.1.  MPLS Backbone . . . . . . . . . . . . . . . . .  9
      4.2.  Configuration of Backbone Equipment . . . . . . . . . . 10
      4.3.  Routing . . . . . . . . . . . . . . . . . . . . . . . . 10
            4.3.1.  IGP . . . . . . . . . . . . . . . . . . . . . . 11
            4.3.2.  EGP . . . . . . . . . . . . . . . . . . . . . . 12
            4.3.3.  Transport of Routing Protocols. . . . . . . . . 12
      4.4.  Multicast . . . . . . . . . . . . . . . . . . . . . . . 13
 5.   Customer Connection Transition Actions. . . . . . . . . . . . 13
      5.1.  Steps in the Transition of Customer Connection Networks 13
            5.1.1.  Small End Sites . . . . . . . . . . . . . . . . 14
            5.1.2.  Large End Sites . . . . . . . . . . . . . . . . 15
      5.2.  User Authentication/Access Control Requirements . . . . 15
      5.3.  Configuration of Customer Equipment . . . . . . . . . . 16
      5.4.  Requirements for Traceability . . . . . . . . . . . . . 16
      5.5.  Ingress Filtering in the Customer Connection Network. . 17
      5.6.  Multihoming . . . . . . . . . . . . . . . . . . . . . . 17
      5.7.  Quality of Service. . . . . . . . . . . . . . . . . . . 17
 6.   Network and Service Operation Actions . . . . . . . . . . . . 18
 7.   Future Stages . . . . . . . . . . . . . . . . . . . . . . . . 18
 8.   Requirements for Follow-On Work . . . . . . . . . . . . . . . 19
 9.   Example Networks. . . . . . . . . . . . . . . . . . . . . . . 19
      9.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . 21
      9.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . 22
      9.3.  Example 3 . . . . . . . . . . . . . . . . . . . . . . . 23
 10.  Security Considerations . . . . . . . . . . . . . . . . . . . 23
 11.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
 12.  Informative References. . . . . . . . . . . . . . . . . . . . 24
      Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . 26
      Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 27
      Full Copyright Statement. . . . . . . . . . . . . . . . . . . 28

1. Introduction

1.1. Goal and Scope of the Document

 When an ISP deploys IPv6, its goal is to provide IPv6 connectivity
 and global address space to its customers.  The new IPv6 service must
 be added to an existing IPv4 service, and the introduction of IPv6
 must not interrupt this IPv4 service.
 An ISP offering IPv4 service will find different ways to add IPv6 to
 this service.  This document discusses a small set of scenarios for
 the introduction of IPv6 into an ISP's IPv4 network.  It evaluates
 the relevance of the existing transition mechanisms in the context of
 these deployment scenarios and points out the lack of essential
 functionality in these methods.

Lind, et al. Informational [Page 2] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 The document is focused on services that include both IPv6 and IPv4
 and does not cover issues surrounding IPv6-only service.  It is also
 outside the scope of this document to describe different types of
 access or network technologies.

2. Brief Description of a Generic ISP Network

 A generic network topology for an ISP can be divided into two main
 parts: the backbone network and customer connection networks.  In
 addition, it includes building blocks such as network and service
 operations.  The additional building blocks used in this document are
 defined as follows:
 "CPE"         : Customer Premises Equipment
 "PE"          : Provider Edge Equipment
 "Network and service operation"
               : This is the part of the ISP's network that hosts the
                 services required for the correct operation of the
                 ISP's network.  These services usually include
                 management, supervision, accounting, billing, and
                 customer management applications.
 "Customer connection"
               : This is the part of the network used by a customer
                 when connecting to an ISP's network.  It includes the
                 CPE, the last hop link, and the parts of the PE
                 interfacing to the last hop link.
 "Backbone"    : This is the rest of the ISP's network infrastructure.
                 It includes the parts of the PE interfacing to the
                 core, the core routers of the ISP, and the border
                 routers used to exchange routing information with
                 other ISPs (or other administrative entities).
 "Dual-stack network"
               : A network that natively supports both IPv4 and IPv6.
 In some cases (e.g., incumbent national or regional operators), a
 given customer connection network may have to be shared between or
 among different ISPs.  According to the type of customer connection
 network used (e.g., one involving only layer 2 devices or one
 involving non-IP technology), this constraint may result in
 architectural considerations relevant to this document.
 The basic components in the ISP's network are depicted in Figure 1.

Lind, et al. Informational [Page 3] RFC 4029 ISP Networks IPv6 Scenarios March 2005

  1. ———– ———-

| Network and| | |

     |  Service   |--| Backbone |
     | Operation  |  |          |\
      ------------    ----------  \
                       / |  \      \
                      /  |   \      \_Peering (Direct and
                     /   |    \                exchange points)
                    /    |     \
                   /     |      \
   ----------     /   ---------- \     ----------
  | Customer |   /   | Customer | \   | Customer |
  |Connection|--/    |Connection|  \--|Connection|
  |     1    |       |     2    |     |     3    |
   ----------         ----------       ----------
        |                  |               |         ISP's Network
        |                  |               |     Customers' Networks
   +--------+        +--------+      +--------+
   |        |        |        |      |        |
   |Customer|        |Customer|      |Customer|
   |        |        |        |      |        |
   +--------+        +--------+      +--------+
                    Figure 1: ISP Network Topology

3. Transition Scenarios

3.1. Identification of Stages and Scenarios

 This section describes different stages an ISP might consider when
 introducing IPv6 connectivity into its existing IPv4 network and the
 different scenarios of what might occur in the respective stages.
 The stages here are snapshots of the ISP's network with respect to
 IPv6 maturity.  Because the ISP's network is continually evolving, a
 stage is a measure of how far along the ISP has come in terms of
 implementing the functionality necessary to offer IPv6 to its
 It is possible for a transition to occur freely between different
 stages.  Although a network segment can only be in one stage at a
 time, the ISP's network as a whole can be in different stages.
 Different transition paths can be followed from the first to the
 final stage.  The transition between two stages does not have to be
 instantaneous; it can occur gradually.

Lind, et al. Informational [Page 4] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 Each stage has different IPv6 properties.  Therefore, based on its
 requirements, an ISP can decide which set of stages it will follow
 and in what order to transform its network.
 This document is not aimed at covering small ISPs, hosting providers,
 or data centers; only the scenarios applicable to ISPs eligible for
 at least a /32 IPv6 prefix allocation from an RIR are covered.

3.2. Stages

 The stages are derived from the generic description of an ISP's
 network in Section 2.  Combinations of different building blocks that
 constitute an ISP's environment lead to a number of scenarios from
 which the ISP can choose.  The scenarios most relevant to this
 document are those that maximize an ISP's ability to offer IPv6 to
 its customers in the most efficient and feasible way.  The assumption
 in all stages is that the ISP's goal is to offer both IPv4 and IPv6
 to the customer.
 The four most probable stages are as follows:
       o Stage 1      Launch
       o Stage 2a     Backbone
       o Stage 2b     Customer connection
       o Stage 3      Complete
 Generally, an ISP is able to upgrade a current IPv4 network to an
 IPv4/IPv6 dual-stack network via Stage 2b, but the IPv6 service can
 also be implemented at a small cost by adding simple tunnel
 mechanisms to the existing configuration.  When a new network is
 designed, Stage 3 might be the first or last step because there are
 no legacy concerns.  Nevertheless, the absence of IPv6 capability in
 the network equipment can still be a limiting factor.
 Note that in every stage except Stage 1, the ISP can offer both IPv4
 and IPv6 services to its customers.

3.2.1. Stage 1 Scenarios: Launch

 The first stage is an IPv4-only ISP with an IPv4 customer.  This is
 the most common case today and is the natural starting point for the
 introduction of IPv6.  From this stage, the ISP can move (undergo a
 transition) from Stage 1 to any other stage with the goal of offering
 IPv6 to its customer.
 The immediate first step consists of obtaining a prefix allocation
 (typically a /32) from the appropriate RIR (e.g., AfriNIC, APNIC,
 ARIN, LACNIC, RIPE) according to allocation procedures.

Lind, et al. Informational [Page 5] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 The ISP will also need to establish IPv6 connectivity to its upstream
 providers and peers; it is of utmost importance to require IPv6
 transit when negotiating IP transit deals with the upstream ISPs.  If
 the upstream is not providing IPv6 connectivity at the moment, it may
 be possible to obtain temporary connectivity from a nearby ISP,
 possibly using a short configured tunnel.  However, the longer-term
 goal must be to require and to obtain IPv6 connectivity from the
 transit ISPs, because otherwise the quality of IPv6 connectivity will
 likely be poor.
 Connectivity to peers can typically be established either directly or
 at Internet Exchange Points (IX).  Most IXs use techniques where IPv6
 is easy to use, and many IXs already provide infrastructure for IPv6
 peerings.  Such peerings can be done natively by using IPv6.
 Peerings over IPv6-in-IPv4 tunnels is also possible but not
 recommended, at least in the long term.  Direct connectivity to peers
 may be feasible when there is direct connectivity to the peer for

3.2.2. Stage 2a Scenarios: Backbone

 Stage 2a deals with an ISP with IPv4-only customer connection
 networks and a backbone that supports both IPv4 and IPv6.  In
 particular, the ISP has the possibility of making the backbone IPv6-
 capable through software upgrades, hardware upgrades, or a
 combination of both.
 Since the customer connections have not yet been upgraded, a
 tunneling mechanism has to be used to provide IPv6 connectivity
 through the IPv4 customer connection networks.  The customer can
 terminate the tunnel at the CPE (if it has IPv6 support) or at some
 set of devices internal to its network.  That is, either the CPE or a
 device inside the network could provide global IPv6 connectivity to
 the rest of the devices in the customer's network.

3.2.3. Stage 2b Scenarios: Customer Connection

 Stage 2b consists of an ISP with an IPv4 backbone network and a
 customer connection network that supports both IPv4 and IPv6.
 Because the service to the customer is native IPv6, the customer is
 not required to support both IPv4 and IPv6.  This is the biggest
 difference from the previous stage.  The need to exchange IPv6
 traffic still exists but might be more complicated than in the
 previous case because the backbone is not IPv6-enabled.  After
 completing Stage 2b, the original IPv4 backbone is unchanged.  This
 means that the IPv6 traffic is transported either by tunneling over
 the existing IPv4 backbone, or in an IPv6 overlay network more or
 less separated from the IPv4 backbone.

Lind, et al. Informational [Page 6] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 Normally, the ISP will continue to provide IPv4 connectivity by using
 private (NATted by the ISP) or public IPv4 address.  In many cases,
 the customer also has a NAT of his/her own; if so, this likely
 continues to be used for IPv4 connectivity.

3.2.4. Stage 3 Scenarios: Complete

 Stage 3 could be considered the final step in introducing IPv6, at
 least within the scope of this document.  This stage consists of
 ubiquitous IPv6 service with native support for IPv6 and IPv4 in both
 backbone and customer connection networks.  From the customer's
 perspective, it is identical to the previous stage because the
 customer connection network has not changed.  The requirement for
 exchanging IPv6 traffic is identical to that of Stage 2.

3.2.5. Stages 2a and 3: Combination Scenarios

 Some ISPs may use different access technologies of varying IPv6
 maturity.  This may result in a combination of the Stages 2a and 3:
 some customer connections do not support IPv6, but others do; in both
 cases the backbone is dual-stack.
 This scenario is equivalent to Stage 2a, but it requires support for
 native IPv6 customer connections on some access technologies.

3.3. Transition Scenarios

 Given the different stages, it is clear that an ISP has to be able to
 make a transition from one stage to another.  The initial stage in
 this document is an IPv4-only service and network.  The end stage is
 a dual IPv4/IPv6 service and network.
 The transition starts with an IPv4 ISP and then moves in one of three
 directions.  This choice corresponds to the different transition
 scenarios.  Stage 2a consists of upgrading the backbone first.  Stage
 2b consists of upgrading the customer connection network.  Finally,
 Stage 3 consists of introducing IPv6 in both the backbone and
 customer connections as needed.
 Because most ISP backbone IPv4 networks continually evolve (firmware
 replacements in routers, new routers, etc.), they can be made ready
 for IPv6 without additional investment (except staff training).  This
 transition path may be slower but still useful, as it allows for the
 introduction of IPv6 without any actual customer demand.  This
 approach may be superior to doing everything at the last minute,
 which may entail a higher investment.  However, it is important to
 consider (and to request from vendors) IPv6 features in all new
 equipment from the outset.  Otherwise, the time and effort required

Lind, et al. Informational [Page 7] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 to remove non-IPv6-capable hardware from the network may be

3.4. Actions Needed When Deploying IPv6 in an ISP's Network

 Examination of the transitions described above reveals that it is
 possible to split the work required for each transition into a small
 set of actions.  Each action is largely independent of the others,
 and some actions may be common to multiple transitions.
 Analysis of the possible transitions leads to a small list of
  • Actions required for backbone transition:
  1. Connect dual-stack customer connection networks to other

IPv6 networks through an IPv4 backbone.

  1. Transform an IPv4 backbone into a dual-stack one. This

action can be performed directly or through intermediate

  • Actions required for customer connection transition:
  1. Connect IPv6 customers to an IPv6 backbone through an IPv4


  1. Transform an IPv4 customer connection network into a dual-

stack one.

  • Actions required for network and service operation transition:
  1. Set up IPv6 connectivity to upstream providers and peers.
  1. Configure IPv6 functions into network components.
  1. Upgrade regular network management and monitoring

applications to take IPv6 into account.

  1. Extend customer management (e.g., RADIUS) mechanisms to be

able to supply IPv6 prefixes and other information to

  1. Enhance accounting, billing, and so on to work with IPv6 as

needed. (Note: If dual-stack service is offered, this may

          not be necessary.)
  1. Implement security for network and service operation.

Lind, et al. Informational [Page 8] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 Sections 4, 5, and 6 contain detailed descriptions of each action.

4. Backbone Transition Actions

4.1. Steps in the Transition of Backbone Networks

 In terms of physical equipment, backbone networks mainly consist of
 high-speed core and edge routers.  Border routers provide peering
 with other providers.  Filtering, routing policy, and policing
 functions are generally managed on border routers.
 In the beginning, an ISP has an IPv4-only backbone.  In the end, the
 backbone is completely dual-stack.  In between, intermediate steps
 may be identified:
                   Tunnels         Tunnels        Dual        Full
 IPv4-only ---->      or      --->   or         + Stack --> Dual Stack
                dedicated IPv6   dedicated IPv6  routers
                    links           links
                      Figure 2: Transition Path
 The first step involves tunnels or dedicated links but leaves
 existing routers unchanged.  Only a small set of routers then have
 IPv6 capabilities.  The use of configured tunnels is adequate during
 this step.
 In the second step, some dual-stack routers are added, progressively,
 to this network.
 The final step is reached when all or almost all routers are
 For many reasons (technical, financial, etc.), the ISP may progress
 step by step or jump directly to the final one.  One important
 criterion in planning this evolution is the number of IPv6 customers
 the ISP expects during its initial deployments.  If few customers
 connect to the original IPv6 infrastructure, then the ISP is likely
 to remain in the initial steps for a long time.
 In short, each intermediate step is possible, but none is mandatory.

4.1.1. MPLS Backbone

 If MPLS is already deployed in the backbone, it may be desirable to
 provide IPv6-over-MPLS connectivity.  However, setting up an IPv6
 Label Switched Path (LSP) requires signaling through the MPLS

Lind, et al. Informational [Page 9] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 network; both LDP and RSVP-TE can set up IPv6 LSPs, but this might
 require upgrade/change in the MPLS core network.
 An alternative approach is to use BGP for signaling or to perform;
 for example, IPv6-over-IPv4/MPLS, as described in [BGPTUNNEL].  Some
 possibilities are preferable to others, depending on the specific
 environment under consideration.  The approaches seem to be as
       1) Require that MPLS networks deploy native IPv6 routing and
          forwarding support.
       2) Require that MPLS networks support native routing and
          setting up of IPv6 LSPs, used for IPv6 connectivity.
       3) Use only configured tunneling over IPv4 LSPs.
       4) Use [BGPTUNNEL] to perform IPv6-over-IPv4/MPLS encapsulation
          for IPv6 connectivity.
 Approaches 1) and 2) are clearly the best target approaches.
 However, approach 1) may not be possible if the ISP is not willing to
 add IPv6 support in the network, or if the installed equipment is not
 capable of high performance native IPv6 forwarding.  Approach 2) may
 not be possible if the ISP is unwilling or unable to add IPv6 LSP
 set-up support in the MPLS control plane.
 Approach 4) can be used as an interim mechanism when other options
 are unfeasible or undesirable for the reasons discussed above.
 Approach 3) is roughly equivalent to approach 4) except that it does
 not require additional mechanisms but may lack scalability in the
 larger networks, especially if IPv6 is widely deployed.

4.2. Configuration of Backbone Equipment

 In the backbone, the number of devices is small, and IPv6
 configuration mainly deals with routing protocol parameters,
 interface addresses, loop-back addresses, access control lists, and
 so on.
 These IPv6 parameters need to be configured manually.

4.3. Routing

 ISPs need routing protocols to advertise reachability and to find the
 shortest working paths, both internally and externally.

Lind, et al. Informational [Page 10] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 Either OSPFv2 or IS-IS is typically used as the IPv4 IGP.  RIPv2 is
 not usually used in service provider networks, as OSPF and IS-IS are
 superior IGPs.  BGP is the only IPv4 EGP.  Static routes also are
 used in both cases.
 Note that it is possible to configure a given network so that it has
 an IPv6 topology different from its IPv4 topology.  For example, some
 links or interfaces may be dedicated to IPv4-only or IPv6-only
 traffic, or some routers may be dual-stack whereas others may be
 IPv4- or IPv6-only.  In this case, routing protocols must be able to
 understand and cope with multiple topologies.

4.3.1. IGP

 Once the IPv6 topology has been determined, the choice of IPv6 IGP
 must be made: either OSPFv3 or IS-IS for IPv6.  RIPng is not
 appropriate in most contexts, due to RIPv2 not being appropriate for
 IPv4 either, and is therefore not discussed here.  The IGP typically
 includes the routers' point-to-point and loop-back addresses.
 The most important decision is whether one wishes to have separate
 routing protocol processes for IPv4 and IPv6.  Separating them
 requires more memory and CPU for route calculations, e.g., when the
 links flap.  But separation provides a measure of assurance that
 should problems arise with IPv6 routing, they will not affect the
 IPv4 routing protocol.  In the initial phases, if it is uncertain
 whether joint IPv4-IPv6 networking is working as intended, running
 separate processes may be desirable and easier to manage.
 The possible combinations are as follows:
  1. With separate processes:

o OSPFv2 for IPv4, IS-IS for IPv6 (only)

       o OSPFv2 for IPv4, OSPFv3 for IPv6, or
       o IS-IS for IPv4, OSPFv3 for IPv6
  1. With the same process:

o IS-IS for both IPv4 and IPv6

 Note that if IS-IS is used for both IPv4 and IPv6, the IPv4/IPv6
 topologies must be "convex", unless the multiple-topology IS-IS
 extensions [MTISIS] have been implemented (using IS-IS for only IPv4
 or only IPv6 requires no convexity).  In simpler networks or with
 careful planning of IS-IS link costs, it is possible to keep even
 incongruent IPv4/IPv6 topologies "convex".  The convexity problem is
 explained in more detail with an example in Appendix A.

Lind, et al. Informational [Page 11] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 When deploying full dual-stack in the short-term, using single-
 topology IS-IS is recommended.  This may be particularly applicable
 for some larger ISPs.  In other scenarios, choosing between one or
 two separate processes often depends on the perceived risk to the
 IPv4 routing infrastructure, i.e., whether one wishes to keep them
 separate for the time being.  If this is not a factor, using a single
 process is usually preferable for operational reasons: not having to
 manage two protocols and topologies.
 The IGP is typically only used to carry loopback and point-to-point
 addresses and doesn't include customer prefixes or external routes.
 Internal BGP (iBGP), as described in the next section, is most often
 deployed in all routers (PE and core) to distribute routing
 information about customer prefixes and external routes.
 Some of the simplest devices (e.g., CPE routers) may not implement
 routing protocols other than RIPng.  In some cases, therefore, it may
 be necessary to run RIPng in addition to one of the above IGPs, at
 least in a limited fashion, and then, by some mechanism, to
 redistribute routing information between the routing protocols.

4.3.2. EGP

 BGP is used for both internal and external BGP sessions.
 BGP with multiprotocol extensions [RFC2858] can be used for IPv6
 [RFC2545].  These extensions enable the exchange of IPv6 routing
 information and the establishment of BGP sessions using TCP over
 It is possible to use a single BGP session to advertise both IPv4 and
 IPv6 prefixes between two peers.  However, the most common practice
 today is to use separate BGP sessions.

4.3.3. Transport of Routing Protocols

 IPv4 routing information should be carried by IPv4 transport and,
 similarly, IPv6 routing information by IPv6 for several reasons:
  • IPv6 connectivity may work when IPv4 connectivity is down (or


  • The best route for IPv4 is not always the best one for IPv6.
  • The IPv4 and IPv6 logical topologies may be different because

the administrator may want to assign different metrics to a

       physical link for load balancing or because tunnels may be in

Lind, et al. Informational [Page 12] RFC 4029 ISP Networks IPv6 Scenarios March 2005

4.4. Multicast

 Currently, IPv6 multicast is not a major concern for most ISPs.
 However, some of them are considering deploying it.  Multicast is
 achieved by using the PIM-SM and PIM-SSM protocols.  These also work
 with IPv6.
 Information about multicast sources is exchanged by using MSDP in
 IPv4, but MSDP is intentionally not defined for IPv6.  Instead, one
 should use only PIM-SSM or an alternative mechanism for conveying the
 information [EMBEDRP].

5. Customer Connection Transition Actions

5.1. Steps in the Transition of Customer Connection Networks

 Customer connection networks are generally composed of a small set of
 PEs connected to a large set of CPEs and may be based on different
 technologies depending on the customer type or size, as well as the
 required bandwidth or even quality of service.  Small unmanaged
 connection networks used for public customers usually rely on
 different technologies (e.g., dial-up or DSL) than the ones used for
 large customers, which typically run managed networks.  Transitioning
 these infrastructures to IPv6 can be accomplished in several steps,
 but some ISPs, depending on their perception of the risks, may avoid
 some of the steps.
 Connecting IPv6 customers to an IPv6 backbone through an IPv4 network
 can be considered a first careful step taken by an ISP to provide
 IPv6 services to its IPv4 customers.  Some ISPs may also choose to
 provide IPv6 service independently from the regular IPv4 service.
 In any case, IPv6 service can be provided by using tunneling
 techniques.  The tunnel may terminate at the CPE corresponding to the
 IPv4 service or in some other part of the customer's infrastructure
 (for instance, on IPv6-specific CPE or even on a host).
 Several tunneling techniques have already been defined: configured
 tunnels with tunnel broker, 6to4 [RFC3056], Teredo [TEREDO], and so
 on.  Some of these are based on a specific addressing plan
 independent of the ISP's allocated prefix(es), while others use a
 part of the ISP's prefix.  In most cases, using the ISP's address
 space is preferable.
 A key factor is the presence or absence of NATs between the two
 tunnel end-points.  In most cases, 6to4 and ISATAP are incompatible
 with NATs, and UDP encapsulation for configured tunnels has not been

Lind, et al. Informational [Page 13] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 Dynamic and non-permanent IPv4 address allocation is another factor a
 tunneling technique may have to deal with.  In this case, the
 tunneling techniques may be more difficult to deploy at the ISP's
 end, especially if a protocol including authentication (like PPP for
 IPv6) is not used.  This may need to be considered in more detail.
 However, NAT traversal can be avoided if the NAT supports forwarding
 protocol-41 [PROTO41] and is configured to do so.
 Firewalls in the path can also break tunnels of these types.  The
 administrator of the firewall needs to create a hole for the tunnel.
 This is usually manageable, as long as the firewall is controlled by
 either the customer or the ISP, which is almost always the case.
 When the CPE is performing NAT or firewall functions, terminating the
 tunnels directly at the CPE typically simplifies the scenario
 considerably, avoiding the NAT and firewall traversal.  If such an
 approach is adopted, the CPE has to support the tunneling mechanism
 used, or be upgraded to do so.

5.1.1. Small End Sites

 Tunneling considerations for small end sites are discussed in
 [UNMANEVA].  These identify solutions relevant to the first category
 of unmanaged networks.  The tunneling requirements applicable in
 these scenarios are described in [TUNREQS].
 The connectivity mechanisms can be categorized as "managed" or
 "opportunistic".  The former consist of native service or a
 configured tunnel (with or without a tunnel broker); the latter
 include 6to4 and, e.g., Teredo -- they provide "short-cuts" between
 nodes using the same mechanisms and are available without contracts
 with the ISP.
 The ISP may offer opportunistic services, mainly a 6to4 relay,
 especially as a test when no actual service is offered yet.  At the
 later phases, ISPs might also deploy 6to4 relays and Teredo servers
 (or similar) to optimize their customers' connectivity to 6to4 and
 Teredo nodes.
 Opportunistic services are typically based on techniques that don't
 use IPv6 addresses from the ISP's allocated prefix(es), and the
 services have very limited functions to control the origin and the
 number of customers connected to a given relay.
 Most interesting are the managed services.  When dual-stack is not an
 option, a form of tunneling must be used.  When configured tunneling
 is not an option (e.g., due to dynamic IPv4 addressing), some form of

Lind, et al. Informational [Page 14] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 automation has to be used.  Basically, the options are either to
 deploy an L2TP architecture (whereby the customers would run L2TP
 clients and PPP over it to initiate IPv6 sessions) or to deploy a
 tunnel configuration service.  The prime candidates for tunnel
 configuration are STEP [STEP] and TSP [TSP], which both also work in
 the presence of NATs.  Neither is analyzed further in this document.

5.1.2. Large End Sites

 Large end sites usually have a managed network.
 Dual-stack access service is often a possibility, as the customer
 network is managed (although CPE upgrades may be necessary).
 Configured tunnels, as-is, are a good solution when a NAT is not in
 the way and the IPv4 end-point addresses are static.  In this
 scenario, NAT traversal is not typically required.  If fine-grained
 access control is needed, an authentication protocol needs to be
 Tunnel brokering solutions have been proposed to help facilitate the
 set-up of a bi-directional tunnel.  Such mechanisms are typically
 unnecessary for large end-sites, as simple configured tunneling or
 native access can be used instead.  However, if such mechanisms would
 already be deployed, large sites starting to deploy IPv6 might
 benefit from them in any case.
 Teredo is not applicable in this scenario, as it can only provide
 IPv6 connectivity to a single host, not the whole site.  6to4 is not
 recommended due to its reliance on the relays and provider-
 independent address space, which makes it impossible to guarantee the
 required service quality and manageability large sites typically

5.2. User Authentication/Access Control Requirements

 User authentication can be used to control who can use the IPv6
 connectivity service in the first place or who can access specific
 IPv6 services (e.g., NNTP servers meant for customers only).  The
 former is described at more length below.  The latter can be achieved
 by ensuring that for all the service-specific IPv4 access lists,
 there are also equivalent IPv6 access lists.
 IPv6-specific user authentication is not always required.  An example
 would be a customer of the IPv4 service automatically having access
 to the IPv6 service.  In this case, the IPv4 access control also
 provides access to the IPv6 services.

Lind, et al. Informational [Page 15] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 When a provider does not wish to give its IPv4 customers automatic
 access to IPv6 services, specific IPv6 access control must be
 performed parallel with the IPv4 access control.  This does not imply
 that different user authentication must be performed for IPv6, but
 merely that the authentication process may lead to different results
 for IPv4 and IPv6 access.
 Access control traffic may use IPv4 or IPv6 transport.  For instance,
 RADIUS [RFC2865] traffic related to IPv6 service can be transported
 over IPv4.

5.3. Configuration of Customer Equipment

 The customer connection networks are composed of PE and CPE(s).
 Usually, each PE connects multiple CPE components to the backbone
 network infrastructure.  This number may reach tens of thousands of
 customers, or more.  The configuration of CPE is difficult for the
 ISP, and it is even more difficult when it must be done remotely.  In
 this context, the use of auto-configuration mechanisms is beneficial,
 even if manual configuration is still an option.
 The parameters that usually need to be provided to customers
 automatically are as follows:
  1. The network prefix delegated by the ISP
  2. The address of the Domain Name System server (DNS)
  3. Possibly other parameters (e.g., the address of an NTP


 When user identification is required on the ISP's network, DHCPv6 may
 be used to provide configurations; otherwise, either DHCPv6 or a
 stateless mechanism may be used.  This is discussed in more detail in
 Note that when the customer connection network is shared between the
 users or the ISPs and is not just a point-to-point link,
 authenticating the configuration of the parameters (especially prefix
 delegation) requires further study.
 As long as IPv4 service is available alongside IPv6, it is not
 required to auto configure IPv6 parameters in the CPE, except the
 prefix, because the IPv4 settings may be used.

5.4. Requirements for Traceability

 Most ISPs have some kind of mechanism to trace the origin of traffic
 in their networks.  This also has to be available for IPv6 traffic,
 meaning that a specific IPv6 address or prefix has to be tied to a

Lind, et al. Informational [Page 16] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 certain customer, or that records must be maintained of which
 customer had which address or prefix.  This also applies to the
 customers with tunneled connectivity.
 This can be done, for example, by mapping a DHCP response to a
 physical connection and storing the result in a database.  It can
 also be done by assigning a static address or prefix to the customer.
 A tunnel server could also provide this mapping.

5.5. Ingress Filtering in the Customer Connection Network

 Ingress filtering must be deployed toward the customers, everywhere,
 to ensure traceability, to prevent DoS attacks using spoofed
 addresses, to prevent illegitimate access to the management
 infrastructure, and so on.
 Ingress filtering can be done, for example, by using access lists or
 Unicast Reverse Path Forwarding (uRPF).  Mechanisms for these are
 described in [RFC3704].

5.6. Multihoming

 Customers may desire multihoming or multi-connecting for a number of
 reasons [RFC3582].
 Mechanisms for multihoming to more than one ISP are still under
 discussion.  One working model would deploy at least one prefix per
 ISP and choose the prefix from the ISP to which traffic is sent.  In
 addition, tunnels may be used for robustness [RFC3178].  Currently,
 there are no provider-independent addresses for end-sites.  Such
 addresses would enable IPv4-style multihoming, with associated
 Multi-connecting more than once to one ISP is a simple practice, and
 this can be done, for example, by using BGP with public or private AS
 numbers and a prefix assigned to the customer.

5.7. Quality of Service

 In most networks, quality of service in one form or another is
 Naturally, the introduction of IPv6 should not impair existing
 Service Level Agreements (SLAs) or similar quality assurances.
 During the deployment of the IPv6 service, the service could be best
 effort or similar, even if the IPv4 service has an SLA.  In the end,
 both IP versions should be treated equally.

Lind, et al. Informational [Page 17] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 IntServ and DiffServ are equally applicable to IPv6 and IPv4 and work
 similarly regardless of IP version.  Of the two, typically only
 DiffServ has been implemented.
 Many bandwidth provisioning systems operate with IPv4 assumptions,
 e.g., taking an IPv4 address or (set of) prefixes for which traffic
 is reserved or preferred.  These systems require special attention
 when introducing IPv6 support in the networks.

6. Network and Service Operation Actions

 The network and service operation actions fall into different
 categories as listed below:
  1. Set up IPv6 connectivity to upstream providers and peers
  2. IPv6 network device configuration: for initial configuration

and updates

  1. IPv6 network management
  2. IPv6 monitoring
  3. IPv6 customer management
  4. IPv6 network and service operation security
 Some of these items will require an available IPv6 native transport
 layer and others will not.
 As a first step, network device configuration and regular network
 management operations can be performed over an IPv4 transport,
 because IPv6 MIBs are also available.  Nevertheless, some monitoring
 functions require the availability of IPv6 transport.  This is the
 case, for instance, when ICMPv6 messages are used by the monitoring
 On many platforms, the current inability to retrieve separate IPv4
 and IPv6 traffic statistics from dual-stack interfaces for management
 purposes by using SNMP is an issue.
 As a second step, IPv6 transport can be provided for any of these
 network and service operation facilities.

7. Future Stages

 At some point, an ISP may want to change to a service that is IPv6
 only, at least in certain parts of its network.  This transition
 creates many new cases into which continued maintenance of the IPv4
 service must be factored.  Providing an IPv6-only service is not much
 different from the dual IPv4/IPv6 service described in stage 3 except
 for the need to phase out the IPv4 service.  The delivery of IPv4
 services over an IPv6 network and the phaseout of IPv4 are issues

Lind, et al. Informational [Page 18] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 left for a subsequent document.  Note that there are some services
 which will need to maintain IPv4 connectivity (e.g., authorative and
 some recursive DNS servers [DNSGUIDE]).

8. Requirements for Follow-On Work

 This section tries to summarize the potential items requiring
 specification in the IETF.
 Work items for which an approach was not yet apparent as of this
 writing are as follows:
  1. A tunnel server/broker mechanism, for the cases where the customer

connection networks cannot be upgraded, needs to be specified

 -  An IPv6 site multihoming mechanism (or multiple ones) needs to be
 Work items which were already fast in progress, as of this writing,
 are as follows:
  1. 6PE for MPLS was identified as a required mechanism, and this is

already in progress [BGPTUNNEL].

  1. IS-IS for Multiple Topologies was noted as a helpful mechanism in

certain environments; however, it is possible to use alternative

    methods to achieve the same end, so specifying this is not
    strictly required.

9. Example Networks

 This section presents a number of different example networks.  These
 will not necessarily match any existing networks but are intended to
 be useful even when they do not correspond to specific target
 networks.  The purpose is to exemplify the applicability of the
 transition mechanisms described in this document to a number of
 different situations with different prerequisites.
 The sample network layout will be the same in each network example.
 This should be viewed as a specific representation of a generic
 network with a limited number of network devices.  A small number of
 routers have been used in the examples.  However, because the network
 examples follow the implementation strategies recommended for the
 generic network scenario, it should be possible to scale the examples
 to fit a network with an arbitrary number, e.g., several hundreds or
 thousands of routers.

Lind, et al. Informational [Page 19] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 The routers in the sample network layout are interconnected with each
 other and with another ISP.  The connection to another ISP can be
 either direct or through an exchange point.  A number of customer
 connection networks are also connected to the routers.  Customer
 connection networks can be, for example, xDSL or cable network
                  ISP1 | ISP2
             +------+  |  +------+
             |      |  |  |      |
             |      |  |  |      |
             +------+  |  +------+
             /      \  +-----------------------
            /        \
           /          \
       +------+    +------+
       |      |    |      |
       |      |    |      |
       +------+    +------+\
           |           |    \             | Exchange point
       +------+    +------+  \  +------+  |  +------+
       |      |    |      |   \_|      |  |  |      |--
       |      |    |      |     |      |  |  |      |--
       +------+   /+------+     +------+  |  +------+
           |     /     |                  |
       +-------+/  +-------+              |
       |       |   |       |
       |Access1|   |Access2|
       |       |   |       |
       +-------+   +-------+
         |||||       |||||  ISP Network
           |           |    Customer Networks
       +--------+  +--------+
       |        |  |        |
       |Customer|  |Customer|
       |        |  |        |
       +--------+  +--------+
             Figure 3: ISP Sample Network Layout

Lind, et al. Informational [Page 20] RFC 4029 ISP Networks IPv6 Scenarios March 2005

9.1. Example 1

 Example 1 presents a network built according to the sample network
 layout with a native IPv4 backbone.  The backbone is running IS-IS
 and IBGP as routing protocols for internal and external routes,
 respectively.  Multiprotocol BGP is used to exchange routes over the
 connections to ISP2 and the exchange point.  Multicast using PIM-SM
 routing is present.  QoS using DiffServ is deployed.
 Access 1 is xDSL connected to the backbone 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 with DHCP.  No routing information is
 exchanged with the customer.  Access control and traceability are
 performed in the access router.  Customers are separated into VLANs
 or separate ATM PVCs up to the access router.
 Access 2 is "fiber to the building or home" (FTTB/H) connected
 directly to the backbone router.  This connection is considered
 layer-3-aware, because it uses layer 3 switches and performs access
 control and traceability through its layer 3 awareness by using DHCP
 snooping.  IPv4 addresses are dynamically assigned to the customers
 with DHCP.  No routing information is exchanged with the customer.
 The actual IPv6 deployment might start by enabling IPv6 on a couple
 of backbone routers, configuring tunnels between them (if not
 adjacent) and connecting to a few peers or upstream providers (either
 through tunnels or at an internet exchange).
 After a trial period, the rest of the backbone is upgraded to dual-
 stack, and IS-IS, without multi-topology extensions (the upgrade
 order is considered with care), is used as an IPv6 and IPv4 IGP.
 During an upgrade until IPv6 customers are connected behind a
 backbone router, the convexity requirement is not critical: The
 routers will just not be reachable with IPv6.  Software supporting
 IPv6 could be installed even though the routers would not be used for
 (customer) IPv6 traffic yet.  That way, IPv6 could be enabled in the
 backbone as needed.
 Separate IPv6 BGP sessions are built similarly to IPv4.  Multicast
 (through SSM and Embedded-RP) and DiffServ are offered at a later
 phase of the network, e.g., after a year of stable IPv6 unicast
 Offering native service as quickly as possible is important.  In the
 meantime, however, a 6to4 relay may be provided in the meantime for
 optimized 6to4 connectivity and may also be combined with a tunnel
 broker for extended functionality.  Operating as bridges at Layer 2

Lind, et al. Informational [Page 21] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 only, xDSL equipment does not require changes in CPE: IPv6
 connectivity can be offered to the customers by upgrading the PE
 router to IPv6.  In the initial phase, only Router Advertisements are
 used; DHCPv6 Prefix Delegation can be added as the next step if no
 other mechanisms are available.
 The FTTB/H access has to be upgraded to support access control and
 traceability in the switches, probably by using DHCP snooping or a
 similar IPv6 capability, but it also has to be compatible with prefix
 delegation, not just address assignment.  This could, however, lead
 to the necessity to use DHCPv6 for address assignment.

9.2. Example 2

 In example 2, the backbone is running IPv4 with MPLS and is using
 OSPF and IBGP for internal and external routes, respectively.  The
 connections to ISP2 and the exchange point run BGP to exchange
 routes.  Multicast and QoS are not deployed.
 Access 1 is a fixed line, e.g., fiber, connected directly to the
 backbone.  Routing information is in some cases exchanged with CPE at
 the customer's site; otherwise static routing is used.  Access 1 can
 also be connected to a BGP/MPLS-VPN running in the backbone.
 Access 2 is xDSL connected directly to the backbone router.  The xDSL
 is layer 2 only, and access control and traceability are achieved
 through PPPoE/PPPoA.  PPP also provides address assignment.  No
 routing information is exchanged with the customer.
 IPv6 deployment might start with an upgrade of a couple of PE routers
 to support [BGPTUNNEL], as this will allow large-scale IPv6 support
 without hardware or software upgrades in the core.  In a later phase,
 native IPv6 traffic or IPv6 LSPs would be used in the whole network.
 In this case, IS-IS or OSPF could be used for the internal routing,
 and a separate IPv6 BGP session would be run.
 For the fixed-line customers, the CPE has to be upgraded, and prefix
 delegation using DHCPv6 or static assignment would be used.  An IPv6
 MBGP session would be used when routing information has to be
 exchanged.  In the xDSL case, the same conditions for IP-tunneling
 apply as in Example 1.  In addition to IP-tunneling,  a PPP session
 can be used to offer IPv6 access to a limited number of customers.
 Later, when clients and servers have been updated, the IPv6 PPP
 session can be replaced with a combined PPP session for both IPv4 and
 IPv6.  PPP has to be used for address and prefix assignment.

Lind, et al. Informational [Page 22] RFC 4029 ISP Networks IPv6 Scenarios March 2005

9.3. Example 3

 A transit provider offers IP connectivity to other providers, but not
 to end users or enterprises.  IS-IS and IBGP are used internally, and
 BGP is used externally.  Its accesses connect Tier-2 provider cores.
 No multicast or QoS is used.
 As this type of transit provider has a number of customers, who have
 a large number of customers in turn, it obtains an address allocation
 from an RIR.  The whole backbone can be upgraded to dual-stack in a
 reasonably short time after a trial with a couple of routers.  IPv6
 routing is performed by using the same IS-IS process and separate
 IPv6 BGP sessions.
 The ISP provides IPv6 transit to its customers for free, as a
 competitive advantage.  It also provides, at the first phase only, a
 configured tunnel service with BGP peering to the significant sites
 and customers (those with an AS number) who are the customers of its
 customers whenever its own customer networks are not offering IPv6.
 This is done both to introduce them to IPv6 and to create a
 beneficial side effect: A bit of extra revenue is generated from its
 direct customers as the total amount of transited traffic grows.

10. Security Considerations

 This document analyzes scenarios and identifies transition mechanisms
 that could be used for the scenarios.  It does not introduce any new
 security issues.  Security considerations of each mechanism are
 described in the respective documents.
 However, a few generic observations are in order.
    o  Introducing IPv6 adds new classes of security threats or
       requires adopting new protocols or operational models than
       those for IPv4; typically these are generic issues, to be
       discussed further in other documents, for example, [V6SEC].
    o  The more complex the transition mechanisms employed become, the
       more difficult it will be to manage or analyze their impact on
       security.  Consequently, simple mechanisms are preferable.
    o  This document has identified a number of requirements for
       analysis or further work that should be explicitly considered
       when adopting IPv6: how to perform access control over shared
       media or shared ISP customer connection media, how to manage
       the configuration management security on such environments

Lind, et al. Informational [Page 23] RFC 4029 ISP Networks IPv6 Scenarios March 2005

       (e.g., DHCPv6 authentication keying), and how to manage
       customer traceability if stateless address autoconfiguration is

11. Acknowledgments

 This document has greatly benefited from input by Marc Blanchet,
 Jordi Palet, Francois Le Faucheur, Ronald van der Pol, and Cleve
 Special thanks to Richard Graveman and Michael Lambert for
 proofreading the document.

12. Informative References

 [EMBEDRP]      Savola, P. and B. Haberman, "Embedding the Rendezvous
                Point (RP) Address in an IPv6 Multicast Address", RFC
                3956, November 2004.
 [MTISIS]       Przygienda, T., Naiming Shen, Nischal Sheth, "M-ISIS:
                Multi Topology (MT) Routing in IS-IS", Work in
 [RFC2858]      Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
                "Multiprotocol Extensions for BGP-4", RFC 2858, June
 [RFC2545]      Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
                Extensions for IPv6 Inter-Domain Routing", RFC 2545,
                March 1999.
 [RFC3704]      Baker, F. and P. Savola, "Ingress Filtering for
                Multihomed Networks", BCP 84, RFC 3704, March 2004.
 [RFC3582]      Abley, J., Black, B., and V. Gill, "Goals for IPv6
                Site-Multihoming Architectures", RFC 3582, August
 [RFC3178]      Hagino, J. and H. Snyder, "IPv6 Multihoming Support at
                Site Exit Routers", RFC 3178, October 2001.
 [RFC3056]      Carpenter, B. and K. Moore, "Connection of IPv6
                Domains via IPv4 Clouds", RFC 3056, February 2001.
 [RFC2865]      Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                "Remote Authentication Dial In User Service (RADIUS)",
                RFC 2865, June 2000.

Lind, et al. Informational [Page 24] RFC 4029 ISP Networks IPv6 Scenarios March 2005

 [BGPTUNNEL]    De Clercq, J., Gastaud, G., Ooms, D., Prevost, S., Le
                Faucheur, F., "Connecting IPv6 Islands across IPv4
                Clouds with BGP", Work in Progress.
 [DUAL-ACCESS]  Shirasaki, Y., Miyakawa, S., Yamasaki, T., Takenouchi,
                A., "A Model of IPv6/IPv4 Dual Stack Internet Access
                Service", Work in Progress.
 [STEP]         Savola, P., "Simple IPv6-in-IPv4 Tunnel Establishment
                Procedure (STEP)", Work in Progress.
 [TSP]          Blanchet, M., "IPv6 Tunnel Broker with Tunnel Setup
                Protocol (TSP)", Work in Progress.
 [TUNREQS]      Palet, J., Nielsen, K., Parent, F., Durand, A.,
                Suryanarayanan, R., and P. Savola, "Goals for
                Tunneling Configuration", Work in Progress, February
 [UNMANEVA]     Huitema, C., Austein, R., Satapati, S., van der Pol,
                R., "Evaluation of Transition Mechanisms for Unmanaged
                Networks", Work in Progress.
 [PROTO41]      Palet, J., Olvera, C., Fernandez, D., "Forwarding
                Protocol 41 in NAT Boxes", Work in Progress.
 [V6SEC]        Savola, P., "IPv6 Transition/Co-existence Security
                Considerations", Work in Progress.
 [DNSGUIDE]     Durand, A., Ihren, J., "DNS IPv6 transport operational
                guidelines", Work in Progress.
 [TEREDO]       Huitema, C., "Teredo: Tunneling IPv6 over UDP through
                NATs", Work in Progress.

Lind, et al. Informational [Page 25] RFC 4029 ISP Networks IPv6 Scenarios March 2005

Appendix A: Convexity Requirements in Single Topology IS-IS

 The single-topology IS-IS convexity requirements could be summarized,
 from IPv4/6 perspective, as follows:
 1) "any IP-independent path from an IPv4 router to any other IPv4
    router must only go through routers which are IPv4-capable", and
 2) "any IP-independent path from an IPv6 router to any other IPv6
    router must only go through routers which are IPv6-capable".
 As IS-IS is based upon CLNS, these are not trivially accomplished.
 The single-topology IS-IS builds paths which are agnostic of IP
 Consider an example scenario of three IPv4/IPv6-capable routers and
 an IPv4-only router:
         cost 5     R4   cost 5
         ,------- [v4/v6] -----.
        /                       \
   [v4/v6] ------ [ v4 ] -----[v4/v6]
     R1   cost 3    R3  cost 3  R2
 Here the second requirement would not hold.  IPv6 packets from R1 to
 R2 (or vice versa) would go through R3, which does not support IPv6,
 and the packets would get discarded.  By reversing the costs between
 R1-R3, R3-R2 and R1-R4,R4-R2 the traffic would work in the normal
 case, but if a link fails and the routing changes to go through R3,
 the packets would start being discarded again.

Lind, et al. Informational [Page 26] RFC 4029 ISP Networks IPv6 Scenarios March 2005

Authors' Addresses

 Mikael Lind
 Vitsandsgatan 9B
 SE-12386 Farsta, Sweden
 Vladimir Ksinant
 Thales Communications
 160, boulevard de Valmy
 92704 Colombes, France
 Soohong Daniel Park
 Mobile Platform Laboratory, SAMSUNG Electronics.
 416, Maetan-3dong, Paldal-Gu,
 Suwon, Gyeonggi-do, Korea
 Alain Baudot
 France Telecom R&D Division
 42, rue des coutures
 14066 Caen - FRANCE
 Pekka Savola
 Espoo, Finland

Lind, et al. Informational [Page 27] RFC 4029 ISP Networks IPv6 Scenarios March 2005

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 retain all their rights.
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Lind, et al. Informational [Page 28]

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