High Availability can be implemented at the service level, the system level, or the network level. While the SWITCH exam requirements are centered around network-level HA, it is important to have an understanding of both service-level and system-level HA. These three types of HA are described in the following sections.
Service-Level High Availability
In Cisco Catalyst switches, modular IOS software provides the ability to upgrade and restart services running on the system without affecting the entire system. Because individual processes or services run in a protected memory space, a failure in one service instance will not affect any other services running at that time. Service-level HA is also supported in the Cisco Nexus platforms, which run NX-OS. Service-level HA ensures that if a less-critical system process fails or is not operating as expected, critical functions required to maintain packet forwarding are not affected.
System-Level High Availability
System-level HA pertains to redundancy within the system itself. Examples of system-level HA include dual power supplies and dual Supervisor Engines within the same physical chassis. System-level HA can also include spare switches and space components for the existing switches. In addition to physical hardware, system-level HA also includes features that allow for fast failover, examples being Stateful Switchover and Non-Stop Forwarding.
Network-Level High Availability
Network-level HA includes both physical and logical components. Physical components include redundant links and devices. Logical components include network monitoring and event logging capabilities, which allow you to respond proactively to network incidents. In addition, protocols that converge quickly, such as RSTP, can be used to facilitate high network availability. Finally, it is also important to remember that the configuration implemented on the switch also affects HA.
The sections that follow describe the various network-level and system-level HA technologies that are integrated into or supported by Cisco IOS software. The chapter will be concluded by describing some of the tools that are used for availability monitoring.