Standby Controller Role in Hitless Stacking

In software releases that do not support hitless stacking, the standby controller functions as a dummy device, meaning it provides limited access to CLI commands such as show, stack, and a few debug commands. The active controller can access the full range of the CLI. The standby controller synchronizes its configuration with the active controller at each reset.

With the introduction of hitless stacking, the standby controller shadows the active controller. The role or behavior of the standby controller with hitless stacking is as follows:

• The local console on the standby controller still accepts only show, stack, and a few debug commands.
• The runtime configuration on the standby controller is synchronized with the active controller whenever there is a configuration change.
• Protocols are configured in the runtime configuration, but no protocol packets are sent out on the standby controller.
• The state of every unit is known, including the state of the active controller. The show commands display current information such as STP or port states.
• When a failover occurs, the standby controller uses its current runtime configuration. The configuration could be different from the active controller if the last configuration transmission was lost.
• After a failover, the new active controller (the old standby controller) programs all other units in hardware, based on its runtime configuration.