Revision as of 06:20, 31 August 2011

Understanding Hign Availability

High availability is technology that enables networkwide resilience to increase IP network availability. Network applications must cross different network segments, from the Enterprise Backbone, Enterprise Edge, and Service Provider Edge, through the Service Provider Core. All segments must be resilient to recover quickly enough for faults to be transparent to users and network applications.

Redundancy

A redundant design can use several mechanisms to prevent single points of failure:

• Geographic diversity and path diversity are often included.
• Dual devices and links are common, as shown in Figure 5-2.
• Dual WAN providers are common.
• Dual data centers are sometimes used, especially for large companies and large e-commerce sites.
• Dual collocation facilities, dual phone central office facilities, and dual power substations can be implemented.

Redundant design must trade off cost versus benefit. It takes time to plan redundancy and verify geographic diversity of service providers. Additional links and equipment cost money to purchase and maintain. These options must be balanced against risks, costs of downtime, and so on.

Technology

Several Cisco routing continuity options, such as Cisco Nonstop Forwarding (NSF) and Stateful Switchover (SSO) exist, and graceful restart capabilities improve availability. Techniques exist to detect failure and trigger failover to a redundant device. These tech- niques include service monitoring for Cisco IOS IP Service Level Agreements (SLA) and Object Tracking.

People

In the Prepare, Plan, Design, Implement, Operate, and Optimize (PPDIOO) methodology, the people component is vitally important, too. Staff work habits and skills can impact high availability. For example, attention to detail enhances high availability, whereas carelessness hurts availability.

Processes

Sound, repeatable processes can lead to high availability. Continual process improvement as part of the PPDIOO methodology plays a role in achieving high availability.

Organizations should build repeatable processes in the following ways:

• By documenting change procedures for repeated changes (for example, Cisco IOS Software upgrades)
• By documenting failover planning and lab testing procedures
• By documenting the network implementation procedure so that the process can be revised and improved the next time components are deployed.

Organizations should use labs appropriately, as follows:

• Lab equipment should accurately reflect the production network.
• Failover mechanisms are tested and understood.
• New code is systematically validated before deployment.

Summary