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IPv6 Operations                                                 T. Chown
Internet-Draft                                 University of Southampton
Expires: January 17, 2005                                  July 19, 2004



     Use of VLANs for IPv4-IPv6 Coexistence in Enterprise Networks
                    draft-chown-v6ops-vlan-usage-01


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Copyright Notice


   Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract


   Ethernet VLANs are quite commonly used in enterprise networks for the
   purposes of traffic segregation.  This document describes how such
   VLANs can be readily used to deploy IPv6 networking in an enterprise,
   including the scenario of early deployment prior to availability of
   IPv6-capable switch-router equipment, where IPv6 may be routed in
   parallel with the existing IPv4 in the enterprise and delivered at
   Layer 2 via VLAN technology.  The IPv6 connectivity to the enterprise
   may or may not enter the site via the same physical link.






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Table of Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Enabling IPv6 per link . . . . . . . . . . . . . . . . . . . .  3
     2.1   IPv6 routing . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2   One VLAN per router interface  . . . . . . . . . . . . . .  4
     2.3   Collapsed VLANs on a single interface  . . . . . . . . . .  4
     2.4   Congruent IPv4 and IPv6 Subnets  . . . . . . . . . . . . .  5
     2.5   IPv6 Addressing  . . . . . . . . . . . . . . . . . . . . .  5
     2.6   Final IPv6 Deployment  . . . . . . . . . . . . . . . . . .  5
   3.  Example VLAN topology  . . . . . . . . . . . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Informative References . . . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8




































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1.  Introduction


   Ethernet VLANs are quite commonly used in enterprise networks for the
   purposes of traffic segregation.  This document describes how such
   VLANs can be readily used to deploy IPv6 networking in an enterprise,
   including the scenario of early deployment prior to availability of
   IPv6-capable switch-router equipment, where IPv6 may be routed in
   parallel with the existing IPv4 in the enterprise and delivered to
   the desired LANs via VLAN technology.


   The IEEE 802.1Q VLAN standard allows separate LANs to be implemented
   over a single bridged LAN, by inserting "Virtual LAN" tagging or
   membership information into Ethernet frames.  Hosts and switches that
   support VLANs effectively allow software-based reconfiguration of
   LANs through configuration of the tagging parameters.  The software
   control means VLANs can be used to alter the LAN infrastructure
   without having to physically alter the wiring between the LAN
   segments and Layer 3 routers.


   Many IPv4 enterprise networks are utilising VLAN technology.  Where a
   site does not have IPv6-capable Layer 2/3 switch-router equipment,
   but VLANs are supported, a simple yet effective method exists to
   gradually introduce IPv6 to some or all of that site's network.


   If such a site wishes to introduce IPv6, it may do so by deploying a
   parallel IPv6 routing infrastructure (which as described below may be
   a single PC-based IPv6 router), and then using VLAN technology to
   "overlay" IPv6 links onto existing IPv4 links.  This can be achieved
   without needing any changes to the IPv4 configuration.


   The IPv6 connectivity to the enterprise may or may not enter the site
   via the same physical link, and may be native or tunneled from the
   external provider to the IPv6 routing equipment.


   This VLAN usage is a solution adopted by a number of sites already,
   and is referenced in our Campus Network IPv6 Transition [2] text.


2.  Enabling IPv6 per link


   The precise method by which IPv6 would be "injected" into the
   existing IPv4 network is implementation specific.  The general
   principle is that the IPv6 router device (e.g.  performing IPv6
   Router Advertisements [1] in the case of stateless autoconfiguration)
   is connected to the target link through the use of VLAN capable Layer
   2 equipment.







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2.1  IPv6 routing


   In a typical scenario, one IPv6 router would be deployed, with both
   an external interface and one or more internal interfaces.  The
   external interface connects to the wider IPv6 internet, and may be
   dual-stack if some tunnel mechanism is used for external
   connectivity, or IPv6-only if a native external connection is
   available.


   By connecting the internal interface(s) directly to a VLAN-capable
   switch, and writing VLAN tags on the packets sent from the internal
   router interface to the switch, VLAN tagging on the switch can be
   used to carry tagged traffic across the internal VLAN-capable site
   infrastructure to IPv6 links that may be dispered widely across the
   site network.


   It is not necessary to do VLAN tagging in all cases.  On some Layer 3
   switches, IPv6 traffic can directly be distributed to specific ports
   by adding them to the same protocol-based VLAN (in this case
   IPv6-based VLANs).


2.2  One VLAN per router interface


   The VLAN marking may be done in different ways.  Some sites may
   prefer to use one router interface per VLAN, e.g.  if there are three
   internal IPv6 links, a PC-based IPv6 router with four Ethernet ports
   could be used, one for the external link and three for the internal
   links.  In such a case one switch port would be needed per link, to
   receive the connectivity from each router port.


   In such a deployment, the IPv6 routing could be cascaded through
   lower tier internal IPv6-only routers.  Here, the internal facing
   ports on the IPv6 edge router may feed other IPv6 routers over
   IPv6-only links which in turn inject the IPv6 connectivity (the /64
   size links and associated Router Advertisements) into the VLANs.


2.3  Collapsed VLANs on a single interface


   Using multiple IPv6 routers and one port per IPv6 link (i.e.  VLAN)
   may be unnecessary.  Many devices now support VLAN tagging based on
   virtual interfaces such that multiple IPv6 VLANs could be assigned
   from one physical router interface port.  Thus it is possible to use
   just one router interface for "aggregated" VLAN trunking from a
   switch.  This is a far more interesting case for a site planning the
   introduction of IPv6 to (part of) its site network.


   This approach is viable while IPv6 traffic load is light.  As traffic
   volume grows, the single collapsed interface could be extended to




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   utilise two or more physical ports, where the capacity of the IPv6
   router device allows it.


2.4  Congruent IPv4 and IPv6 Subnets


   Such a VLAN-based technique can be used to deploy IPv6-only VLANs in
   an enterprise network.  However most enterprises will be interested
   in dual-stack IPv4-IPv6 networking.


   In such a case the IPv6 connectivity may be injected into the
   existing IPv4 VLANs, such that the IPv4 and IPv6 subnets are
   congruent (i.e.  they coincide exactly when superimposed).  Such a
   method may have desirable administrative properties, e.g.  the
   devices in each IPv4 subnet will be in the same IPv6 subnets also.
   This is the method being used in our Campus Network IPv6 Transition
   [2] text.


   Further, IPv6-only devices may be gradually added into the subnet
   without any need to resize the IPv6 subnet (which may hold in effect
   an infinite number of hosts in a /64 in contrast to IPv4 where the
   subnet size is often relatively limited, or kept to a minimum
   possible due to address space usage concerns).  The lack of
   requirement to periodically resize an IPv6 subnet is a useful
   administrative advantage for IPv6.


2.5  IPv6 Addressing


   One site using this VLAN technique has chosen to number its IPv6
   links with the format [Site IPv6 prefix]:[VLAN ID]::/64.  This is not
   a recommended addressing plan, but some sites may wish to consider
   its usage.


2.6  Final IPv6 Deployment


   The VLAN technique for IPv6 deployment offers a more structured
   alternative to opportunistic per-host intra-site tunnelling methods
   such as ISATAP [3].  It has the ability to offer a simple yet
   efficient method for early IPv6 deployment to an enterprise site.


   When the site acquires IPv6-capable switch-router equipment, the
   VLAN-based mathod can still be used for delivery of IPv6 links to
   physical switch interfaces, just as it is commonly today for IPv4
   subnets, but with a common routing infrastructure.









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3.  Example VLAN topology


   The following figure shows how a VLAN topology may be used to
   introduce IPv6 in an enterprise network, using a parallel IPv6
   routing infrastructure and VLAN tagging.



        (Subnet1) (Subnet2) (SubnetN)
             \     /            |
            [Switch1]       [SwitchN]
                \             /
                 \           /
    ( VLAN infrastructure in the enterprise )
                      |
     [ Ethernet switch with VLAN support ]
                      |
            FE/GE w/ VLAN tagging
               [ IPv6-router ]
                      |
          ( External IPv6 Internet )



   Figure 1: IPv6 deployment using VLANs


   In this scenario, the router has one physical port facing towards the
   internal infrastructure, and is using the collapsed VLAN mechanism
   described above.  It may have an additional interface towards the
   external infrastructure.  The router can also function as a
   "one-handed" router.


   A number of VLANs are handled by the internal-facing IPv6 router
   port; the VLANs are seen as logical subinterfaces of the physical
   interface.  Therefore, the router acts as an IPv6 first-hop access
   router to the physical links, separately from the IPv4-first hop
   router.  This technique allows a site to easily "inject" native IPv6
   into all the links where a VLAN-capable infrastructure is available,
   enabling partial or full IPv6 deployment on the wire in a site.


4.  Security Considerations


   There are no additional security considerations particular to this
   method of enabling IPv6 on a link.


   Where the IPv6 connectivity is delivered into the enterprise network
   by a different path from the IPv4 connectivity, care should be given
   that equivalent application of security policy (e.g.  firewalling) is
   made to the IPv6 path.





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5.  Acknowledgements


   The author would like to thank colleagues on the 6NET project, where
   this technique for IPv4-IPv6 coexistence is widely deployed,
   including Janos Mohacsi (Hungarnet), Martin Dunmore and Chris Edwards
   (Lancaster University), Christian Strauf (JOIN Project, University of
   Muenster), Stig Venaas (UNINETT) and Pekka Savola (CSC/FUNET).


6  Informative References


   [1]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
        IP Version 6 (IPv6)", RFC 2461, December 1998.


   [2]  Chown, T., "IPv6 Campus Transition Scenario Description and
        Analysis", draft-chown-v6ops-campus-transition-00 (work in
        progress), July 2004.


   [3]  Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
        Automatic Tunnel Addressing Protocol (ISATAP)",
        draft-ietf-ngtrans-isatap-22 (work in progress), May 2004.



Author's Address


   Tim Chown
   University of Southampton


   Southampton, Hampshire  SO17 1BJ
   United Kingdom


   EMail: tjc@ecs.soton.ac.uk





















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   Internet Society.




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