Request for Comments: 4554
University of Southampton
Use of VLANs for IPv4-IPv6 Coexistence in Enterprise 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 © The Internet Society (2006).
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, which focuses on the scenario of early deployment prior to availability of IPv6-capable switch-router equipment. In this method, 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Enabling IPv6 per Link . . . . . . . . . . . . . . . . . . . . 3 2.1. IPv6 Routing over VLANs . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 6. Informative References . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Configuration Example . . . . . . . . . . . . . . . . 8
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.
It is expected that in the long run, sites migrating to dual-stack networking will either upgrade existing switch-router equipment to support IPv6 or procure new equipment that supports IPv6. If a site already has production routers deployed that support IPv6, the procedures described in this document are not required. In the interim, however, a method is required for early IPv6 adopters that enables IPv6 to be deployed in a structured, managed way to some or all of an enterprise network that currently lacks IPv6 support in its core infrastructure.
The IEEE 802.1Q VLAN standard allows separate LANs to be deployed 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 that 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, in advance of the site's core infrastructure having dual-stack capability.
If such a site wishes to introduce IPv6, it may do so by deploying a parallel IPv6 routing infrastructure (which is likely to be a different platform to the site's main infrastructure equipment, i.e., one that supports IPv6 where the existing equipment does not), 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 VLANs don't need to differentiate between IPv4 and IPv6; the deployment is just dual-stack, as Ethernet is without VLANs.
The IPv4 default route to the VLAN is provided by one (IPv4) router, while the IPv6 default route to the VLAN is provided by a different (IPv6) router. The IPv6 router can provide native IPv6 connectivity to the whole site with just a single physical interface, thanks to VLAN tagging and trunking, as described below.
The IPv6 connectivity to the enterprise may or may not enter the site via the same physical link as the IPv4 traffic, 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, including that of the author.
It should be noted that a parallel infrastructure will require additional infrastructure and thus cost, and will often require a separate link into the site (from an IPv6 provider), quite possibly tunneled, that will require the site's security policy to be applied (e.g., firewalling and intrusion detection). For sites that believe early adoption of IPv6 is important, that price is one they may be quite willing to pay. However, this document focuses on the technical issues of VLAN usage in such a scenario.
2. Enabling IPv6 per Link
The precise method by which IPv6 would be "injected" into the existing IPv4 network is deployment specific. For example, perhaps a site has an IPv4-only router, connected to an Ethernet switch that supports VLANs and a number of hosts connected to that VLAN. Let's further assume that the site has a dozen of these setups that it wishes to IPv6-enable immediately. This could be done by upgrading the twelve routers to support IPv6, and turning IPv6 on those routers. However, this may not be practical for various reasons.
The simplest approach would be to connect an IPv6 router with one interface to an Ethernet switch, and connect that switch to other switches, and then use VLAN tags between the switches and the IPv6 router to "reach" all the IPv4-only subnets from the IPv6 router. Thus, the general principle is that the IPv6 router device (e.g., performing IPv6 Router Advertisements  in the case of stateless autoconfiguration) is connected to the target link through the use of VLAN-capable Layer 2 equipment.
2.1. IPv6 Routing over VLANs
In a typical scenario where connectivity is to be offered to a number of existing IPv6 internal subnets, one IPv6 router could 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.
The internal interface(s) can be connected directly to a VLAN-capable switch. It is then possible to write VLAN tags on the packets sent from the internal router interface based on the target IPv6 link prefix. The VLAN-tagged traffic is then transported across the internal VLAN-capable site infrastructure to the target IPv6 links (which may be dispersed widely across the site network).
Where the IPv6 router is unable to VLAN-tag the packets, a protocol- based VLAN can be created on the VLAN-capable device connected to the IPv6 router, causing IPv6 traffic to be tagged and then redistributed on (congruent) IPv4 subnet links that lie in the same VLAN.
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; for example, if there are three internal IPv6 links, a standard 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 stub links using 64-bit subnet prefixes 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 (trunked) 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 the IPv6 traffic load is light. As traffic volume grows, the single collapsed interface could be extended to 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; for example, the devices in each IPv4 subnet will be in the same IPv6 subnets also. This is the method used at the author's site.
Furthermore, 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 possibly 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. The VLAN tag is 16 bits, so this can work with a typical maximum 48-bit site prefix. Linking the VLAN ID into a site's addressing scheme may not fit topology and aggregation, and thus is not necessarily 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 Intra-Site Automatic Tunnel Addressing Protocol ISATAP . 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 method can still be used for delivery of IPv6 links to physical switch interfaces, just as it is commonly used today for IPv4 subnets, but with a common routing infrastructure.
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.
External IPv6 Internet | | IPv6 Access Router | | Switch-router with VLAN support | | +--------------+----------------+ |Site enterprise infrastructure | | with support for VLANs | +----+--------------------+-----+ | | | | VLAN switch A VLAN switch B | | | | | | Subnet1 Subnet2 Subnet3
Figure 1: IPv6 deployment using VLANs (physical diagram)
In this scenario, the IPv6 access router has one physical port facing toward the internal infrastructure. In this example, it need only be IPv6-enabled, as its purpose is solely to handle IPv6 traffic for the enterprise. The access router has an additional interface facing toward the external infrastructure, which in this example could be dual-stack if the external IPv6 connectivity is via a tunnel to an IPv6 ISP.
A number of VLANs are handled by the internal-facing IPv6 router port; in this case, IPv6 links Subnet1, Subnet2, Subnet3. The VLANs are seen as logical subinterfaces of the physical interface on the IPv6 access router, which is using the "collapsed VLAN" method described above, tagging the inbound traffic with one of three VLAN IDs depending on the target IPv6 Subnet prefix.
The following figure shows how the IPv6 view of the deployment looks; all IPv6 subnets are on-link to the IPv6 access router, whether or not they share the same physical links over the VLAN infrastructure.
External IPv6 Internet | | Site IPv6 Access Router | | | | | | Subnet1 Subnet2 Subnet3
Figure 2: IPv6 view of the deployment (logical view)
In this example, 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.
The author would like to thank colleagues on the 6NET project, where this technique for IPv4-IPv6 coexistence is widely deployed, in particular Pekka Savola (CSC/FUNET), but also including Janos Mohacsi (Hungarnet), Martin Dunmore and Chris Edwards (Lancaster University), Christian Strauf (JOIN Project, University of Muenster), and Stig Venaas (UNINETT).
6. Informative References
 Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.  Templin, F., Gleeson, T., Talwar, M., and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 4214, October 2005.
Appendix A. Configuration Example
This section describes a configuration example for using a computer running the FreeBSD variant of the Berkeley Software Distribution (BSD) operating system as a router to deploy IPv6 networking across a number of IPv6 links on an enterprise (in this case, six links), for a scenario similar to the one described above. Here, the precise configuration may of course vary depending on the existing site VLAN deployment. This section highlights that the VLAN configuration must be manually configured; the support is not "automatic".
In this example, the configuration is for an IPv6 BSD router connected directly to a site's external IPv6 access router. The BSD router has one interface (dc0) toward the site IPv6 access router, and three interfaces (dc1, dc2, dc3) over which the internal routing is performed (the number of interfaces can be varied; three are used here to distribute the traffic load). The IPv6 documentation prefix (2001:db8::/32) is used in the example.
--- Example IPv6 VLAN configuration, FreeBSD ---
# To IPv6 enable a vlan
# 1. Add a new vlan device to cloned_interfaces called vlanX
# 2. Add an ifconfig_vlanX line, the number is the vlan tag ID
# 3. Add vlanX to ipv6_network_interfaces
# 4. Add an ipv6_ifconfig_vlanX line, with a new unique prefix
# 5. Add vlanX to rtadvd_interface
# 6. Add vlanX to ipv6_router_flags
### Interfaces ###
# Bring physical interfaces up
# Create VLan interfaces
cloned_interfaces="vlan0 vlan1 vlan2 vlan3 vlan4 vlan5 vlan6"
# Upstream link to IPv6 Access Router
ifconfig_vlan0="vlan 37 vlandev dc0"
# Downstream interfaces, load balance over interfaces dc1,dc2,dc3 ifconfig_vlan1="vlan 11 vlandev dc1" # Subnet1 ifconfig_vlan2="vlan 17 vlandev dc2" # Subnet2 ifconfig_vlan3="vlan 24 vlandev dc3" # Subnet3 ifconfig_vlan4="vlan 25 vlandev dc1" # Subnet4 ifconfig_vlan5="vlan 34 vlandev dc2" # Subnet5 ifconfig_vlan6="vlan 14 vlandev dc3" # Subnet6
### IPv6 ###
# Enable ipv6
# Define Interfaces
ipv6_network_interfaces="vlan0 vlan1 vlan2 vlan3 vlan4 vlan5 vlan6" # Define addresses ipv6_ifconfig_vlan0="2001:db8:d0:101::2 prefixlen 64" # Uplink ipv6_ifconfig_vlan1="2001:db8:d0:111::1 prefixlen 64" # Subnet1 ipv6_ifconfig_vlan2="2001:db8:d0:112::1 prefixlen 64" # Subnet2 ipv6_ifconfig_vlan3="2001:db8:d0:121::1 prefixlen 64" # Subnet3 ipv6_ifconfig_vlan4="2001:db8:d0:113::1 prefixlen 64" # Subnet4 ipv6_ifconfig_vlan5="2001:db8:d0:114::1 prefixlen 64" # Subnet5 ipv6_ifconfig_vlan6="2001:db8:d0:115::1 prefixlen 64" # Subnet6
# Router advertisements
rtadvd_interfaces="-s vlan0 vlan1 vlan2 vlan3 vlan4 vlan5 vlan6"
### Routing ###
ipv6_router_flags="-N dc0,dc1,dc2,dc3, vlan1,vlan2,vlan3,
--- End of configuration ---
Note that if there was only one internal-facing interface, then again so long as the OS supported VLAN trunking, all the VLAN IDs could be associated to that interface (dc1, for example).
The VLAN IDs need to be managed by the site administrator, but would probably already be assigned for existing IPv4 subnets (ones into which IPv6 is being introduced).
For a large enterprise, a combination of internal tunnels and VLAN usage could be used; the whole site need not be enabled by VLAN tagging alone. This choice is one for the site administrator to make.
Tim Chown University of Southampton Southampton, Hampshire SO17 1BJ United Kingdom EMail: firstname.lastname@example.org
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