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Design Considerations for IPv6 in Campus Networks

This section discusses the three different IPv6 deployment models that can be used in the enterprise campus.

IPv6 Campus Design Considerations

As mentioned earlier, three major deployment models can be used to implement IPv6 support in the enterprise campus environment: the dual-stack model, the hybrid model, and the service block model. The choice of deployment model strongly depends on whether IPv6 switching in hardware is supported in the different areas of the network.

Dual stack is the preferred, most versatile, and highest-performance way to deploy IPv6 in existing IPv4 environments. IPv6 can be enabled wherever IPv4 is commissioned along with the associated features that are required to make IPv6 routable, highly available, and secure. In some cases, IPv6 may not be enabled on a specific interface or device because of the presence of legacy applications or hosts for which IPv6 is not supported. Inversely, IPv6 may be enabled on interfaces and devices for which IPv4 support is no longer necessary.

A key requirement for the deployment of the dual-stack model is that IPv6 switching must be performed in hardware on all switches in the campus. If some areas of the campus network do not support IPv6 switching in hardware, tunneling mechanisms are leveraged to integrate these areas into the IPv6 network. The hybrid model combines a dual-stack approach for IPv6-capable areas of the network with tunneling mechanisms such as Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) and manual IPv6 tunnels where needed.

The hybrid model adapts as much as possible to the characteristics of the existing network infrastructure. Transition mechanisms are selected based on multiple criteria, such as IPv6 hardware capabilities of the network elements, number of hosts, types of applications, location of IPv6 services, and network infrastructure feature support for various transition mechanisms.

The service block model uses a different approach to IPv6 deployment. It centralizes IPv6 as a service, similar to how other services such as voice or guest access can be provided at a central location. The service block model is unique in that it can be deployed as an overlay network without any impact to the existing IPv4 network, and it is completely centralized. This overlay network can be implemented rapidly while allowing for high availability of IPv6 services, QoS capabilities, and restriction of access to IPv6 resources with little or no changes to the existing IPv4 network. As the existing campus network becomes IPv6-capable, the service block model can become decentralized. Connections into the service block are changed from tunnels (ISATAP or manually configured) to dual-stack connections. When all the campus layers are dual-stack capable, the service block can be dismantled and repurposed for other uses.

These three models are not exclusive. Elements from each of these models can be combined to support specific network requirements.

Ultimately, a dual-stack deployment is preferred. The hybrid and service block models are transitory solutions. The models can be leveraged to migrate to a dual stack design in a graceful manner, without a need for forced hardware upgrades throughout the entire campus. From an address-planning standpoint, this means that the IPv6 address plan should be designed to support a complete dual-stack design in the future.

Dual-Stack Model

The dual-stack model deploys IPv4 and IPv6 in parallel without any tunneling or translation between the two protocols. IPv6 is enabled in the access, distribution, and core layers of the campus network. This model makes IPv6 simple to deploy, and is very scalable. No dependencies exist between the IPv4 and IPv6 design, which results in easier implementation and troubleshooting.

Deploying IPv6 in the campus using the dual-stack model offers several advantages over the hybrid and service block models. The primary advantage of the dual-stack model is that it does not require tunneling within the campus network. The dual-stack model runs the two protocols as "ships in the night," meaning that IPv4 and IPv6 run alongside one another and have no dependency on each other to function except that they share network resources. Both IPv4 and IPv6 have independent routing, high availability, QoS, security, and multicast policies. The dual-stack model also offers processing performance advantages, because packets are natively forwarded without having to account for additional encapsulation and lookup overhead.

These advantages make the dual-stack model the preferred deployment model. The stack model requires all switches in the campus to support IPv6 forwarding.

Hybrid Model

The hybrid model strategy is to employ two or more independent transition mechanisms with the same deployment design goals. Flexibility is the key aspect of the hybrid approach. Any combination of transition mechanisms can be leveraged to best fit a given network environment. The hybrid model uses dual stack in all areas of the network where the equipment supports IPv6. Tunneling mechanisms are deployed for areas that do not currently support IPv6 in hardware. These areas can be transitioned to dual stack as hardware is upgraded later.

Various tunneling mechanisms and deployment scenarios can be part of a hybrid model deployment. This section highlights two common scenarios.

The first scenario that may require the use of a hybrid model is when the campus core is not enabled for IPv6. Common reasons why the core layer might not be enabled for IPv6 are either that the core layer does not have hardware-based IPv6 support at all, or has limited IPv6 support but with low performance.

In this scenario, manually configured tunnels are used exclusively from the distribution to aggregation layers. Two tunnels from each switch are used for redundancy and load balancing. From an IPv6 perspective, the tunnels can be viewed as virtual links between the distribution and aggregation layer switches. On the tunnels, routing and IPv6 multicast are configured in the same manner as with a dual-stack configuration.

The scalability of this model is limited, and a dual-stack model is preferred. However, this is a good model to use if the campus core is being upgraded or has plans to be upgraded, and access to IPv6 services is required before the completion of the core upgrade.

The second scenario focuses on the situation where hosts that are located in the campus access layer need to use IPv6 services, but the distribution layer is not IPv6 capable or enabled. The distribution layer switch is most commonly the first Layer 3 gateway for the access layer devices. If IPv6 capabilities are not present in the existing distribution layer switches, the hosts cannot gain access to IPv6 addressing router information (stateless autoconfiguration or Dynamic Host Configuration Protocol [DHCP] for IPv6), and then cannot access the rest of the IPv6-enabled network.

In this scenario, tunneling can be used on the IPv6-enabled hosts to provide access to IPv6 services that are located beyond the distribution layer. For example, the ISATAP tunneling mechanisms on the hosts in the access layer to provide IPv6 addressing and off-link routing. The Microsoft Windows XP and Vista hosts in the access layer must have IPv6 enabled and either a static ISATAP router definition or Domain Name System (DNS) A record entry that is configured for the ISATAP router address.

Using the ISATAP IPv4 address, the hosts establish tunnels to the IPv6-enabled core routers, obtain IPv6 addresses, and tunnel IPv6 traffic across the IPv4 distribution switches to the IPv6 enabled part of the network.

Terminating ISATAP tunnels in the core layer makes the layer appear as an access layer to the IPv6 traffic, which may be undesirable from a high-level design perspective. To avoid the blending of core and access layer functions, the ISATAP can be terminated on a different set of switches, such as the data center aggregation switches.

The main reason to choose the hybrid deployment model is to deploy IPv6 without having to go through an immediate hardware upgrade for parts of the network. It allows switches that have not reached the end of their normal life cycle to remain deployed and avoids the added cost that is associated with upgrading equipment before its time with the sole purpose of enabling IPv6.

Some drawbacks apply to the hybrid model. The use of ISATAP tunnels is not compatible with IPv6 multicast. Therefore, any access or distribution layer blocks that require the use of IPv6 multicast applications should be deployed using the dual-stack model. Manual tunnels support IPv6 multicast and can still be used to carry IPv6 across an IPv4 core. Another drawback of the hybrid model is the added complexity that is associated with tunneling. Considerations that must be accounted for include performance, management, security, scalability, and availability.

Service Block Model

The service block model has several similarities to the hybrid model. The underlying IPv4 network is used as the foundation for the overlay IPv6 network that is being deployed. ISATAP provides access to hosts in the access layer. Manually configured tunnels are utilized from the data center aggregation layer to provide IPv6 access to the applications and services that are located in the data center access layer. IPv4 routing is configured between the core layer and service block switches to allow visibility to the service block switches for terminating IPv6-in-IPv4 tunnels.

The biggest difference with the hybrid model is that the service block model centralizes IPv6 connectivity through a separate redundant pair of switches. The service block deployment model is based on a redundant pair of Cisco Catalyst 6500 series switches with a Cisco Supervisor Engine 32 or Supervisor Engine 720 card. The key to maintaining a highly scalable and redundant configuration in the service block is to ensure that a high-performance switch, supervisor, and modules are used to manage the load of the ISATAP, manually configured tunnels, and dual-stack connections for an entire campus network.

The biggest benefit of this model compared with the hybrid model is that the centralized approach enables you to pace the IPv6 deployment in a very controlled manner.

In essence, the service block model provides control over the pace of IPv6 service introduction by leveraging the following:

  • Per-user or per-VLAN tunnels, or both, can be configured via ISATAP to control the flow of connections and allow for the measurement of IPv6 traffic use.
  • Access on a per-server or per-application basis can be controlled via access lists and routing policies that are implemented on the service block switches. This level of control allows for access to one, a few, or even many IPv6-enabled services, while all other services remain on IPv4 until those services can be upgraded or replaced. This enables a "per-service" deployment of IPv6.
  • The use of separate dual redundant switches in the service block allows for high availability of ISATAP and manually configured tunnels as well as all dual-stack connections.
  • Flexible options allow hosts access to the IPv6-enabled ISP connections, either by allowing a segregated IPv6 connection that is used only for IPv6-based Internet traffic or by providing links to the existing Internet edge connections that have both IPv4 and IPv6 ISP connections.
  • Implementation of the service block model does not disrupt the existing network infrastructure and services. Because of its similarity to the hybrid model, the service block model suffers from the same drawbacks that are associated with the use of tunneling. In addition to those drawbacks, there is the cost that is associated with the service block switches.
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