Redundant Network Design Topologies

Redundant network designs let you meet requirements for network availability by duplicating elements in a network. Redundancy attempts to eliminate any single point of failure on the network. The goal is to duplicate any required component whose failure could disable critical applications. The component could be a core router, a switch, a link between two switches, a channel service unit (CSU), a power supply, a WAN trunk, Internet connectivity, and so on. To enable business survivability after a disaster and offer performance benefits from load sharing, some organizations have completely redundant data centers. Other organizations try to constrain network operational expenses by using a less-comprehensive level of redundancy.

Redundancy can be implemented inside individual campus networks and between layers of the hierarchical model. Implementing redundancy on campus networks can help you meet availability goals for users accessing local services. Redundancy can also be implemented on the edge of the enterprise network to ensure high availability for Internet, extranet, and virtual private network (VPN) access.

NOTE

Because redundancy is expensive to deploy and maintain, you should implement redundant topologies with care. Be sure to select a level of redundancy that matches your customer's requirements for availability and affordability.

Before you select redundant design solutions, you should first analyze the business and technical goals of your customer, as discussed in Part I of this book, "Identifying Your Customer's Needs and Goals." Make sure you can identify critical applications, systems, internetworking devices, and links. Analyze your customer's tolerance for risk and the consequences of not implementing redundancy. Make sure to discuss with your customer the tradeoffs of redundancy versus low cost, and simplicity versus complexity. Redundancy adds complexity to the network topology and to network addressing and routing.

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