Stack Device Roles and Elections
There are three distinct roles played by units that are part of a stack:
Active Controller
The active controller contains the saved and running configuration files for each stack member. The configuration files include the system-level settings for the stack and the interface-level settings for each stack member, as well as MIB counters and port status. The standby controller also has a synchronized copy of the active controller startup-config file in case the active controller fails.
When a stack is formed, the console function for each stack member is automatically redirected to the active controller console. The active controller console port handles all stack management functions, as well as ping activity, Telnet sessions, and TFTP image downloads for every stack member. If you connect to the console port on a stack member that is not the active controller, you are automatically directed through the console of the active controller.
The active controller synchronizes its startup configuration with the standby controller
and the rest of the stack members. You can recover the previous flash configuration
of the standby controller and the stack members by issuing the
stack unconfigure command. For an example of this command and the output generated, refer to
Unconfiguring a stack.
The active controller may reset the rest of the stack members, if necessary. However,
if the active controller itself must be reset because configuration has changed or
new images have loaded, you must use the
reload command..
Standby Controller
In addition to the active controller, another stack member is elected as the standby controller. If the active controller fails, the standby controller takes over based on the following criteria:
- If hitless-failover enable is not configured, the standby reloads all units, including itself. It reloads to become the active controller.
- If hitless-failover enable is configured, the standby immediately takes over the active controller role without reloading any unit.
- If the entire stack reloads and the active controller does not come up, the standby waits for four minutes to take over by reloading every unit, including itself.
The standby controller synchronizes its configuration with the active controller at each reset.
Bootup Role
When a stack unit boots, it boots in a particular role, active controller, standby
controller, or stack member, or as a standalone device. When the bootup role is standby
controller or stack member, the CLI available to the unit is limited to
show and
stack commands. A unit in the role of standby controller or stack member does not act without
instructions from the active controller.
The last line of the
show version output identifies the unit role unless the unit is in standalone mode.
My stack unit ID = 1, bootup role = active My stack unit ID = 3, bootup role = standby
stack unconfigure me command (refer to
Unconfiguring a stack).