Designing High Availability into a Network

Redundant network designs duplicate network links and devices, eliminating single points of failure on the network. The goal is to duplicate components whose failure could disable critical applications.

Because redundancy is expensive to deploy and maintain, redundant topologies should be implemented with care. Redundancy adds complexity to the network topology and to network addressing and routing. The level of redundancy should meet the organization's availability and affordability requirements.

KEY POINT

Before selecting redundant design solutions, analyze the business and technical goals and constraints to establish the required availability and affordability.

Critical applications, systems, internetworking devices, and links must be identified. Analyze the risk tolerance and the consequences of not implementing redundancy, and ensure that you consider the trade-offs of redundancy versus cost and simplicity versus complexity. Duplicate any component whose failure could disable critical applications.

Redundancy is not provided by simply duplicating all links. Unless all devices are completely fault-tolerant, redundant links should terminate at different devices; otherwise, devices that are not fault-tolerant become single points of failure.

KEY POINT

Because many other modules access the Server Farm and Campus Core modules, they typically require higher availability than other modules.

The following types of redundancy may be used in the modules of an enterprise:

■ Device redundancy, including card and port redundancy

■ Redundant physical connections to critical workstations and servers

■ Route redundancy

■ Link redundancy

■ Power redundancy, including redundant power supplies integral to the network devices and redundant power to the building's physical plant

KEY POINT

The key requirement in redundancy is to provide alternative paths for mission-critical applications. Simply making the backbone fault-tolerant does not ensure high availability. For example, if communication on a local segment is disrupted for any reason, that information will not reach the backbone. End-to-end high availability is possible only when redundancy is deployed throughout the internetwork.

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