Hitless Stacking Behavior

Hitless stacking is enabled by default for FastIron release 08.0.20 and later releases. In previous releases, the hitless-failover command must be set to enable in the configuration.

If you install a FastIron 08.0.20 or later image on a new system with no previous configuration, the hitless-failover command is enabled by default.

If you upgrade to FastIron release 08.0.20 or later from a previous version that has hitless-failover enabled, hitless-failover is retained as the default.

If you upgrade to FastIron release 08.0.20 or later on a system with an earlier release that does not have hitless-failover enabled in its configuration, the previous configuration is retained.

Hitless stacking can be disabled using the no hitless-failover enable command. When hitless failover is disabled, the following limitations are in effect:

  • If a failover occurs, every unit in the stack reloads.
  • Manual switchover is not allowed. If the stack switch-over command is entered, the following message appears on the console: 
Switch-over is not allowed. Reason: hitless-failover not configured.
  • Internal switchover resulting from a priority change is blocked until the entire stack is reloaded or hitless stacking is enabled. A priority change triggers an election, but the newly elected winner does not immediately assume its role.
  • During a stack merge, the active controller with the highest priority wins the election and reloads every unit of the losing stack.
Note: Synchronization between the active controller, standby controller, and stack members occurs whether or not hitless stacking is enabled.

If hitless stacking has been disabled, enable hitless stacking failover as described in "Enabling hitless stacking failover." As soon as hitless stacking is enabled, the following behavior takes effect immediately:

  • If a failover occurs, the stack does not reload.
  • Manual switchover (CLI command stack switch-over) is allowed.
  • If a priority change occurred while hitless stacking was disabled and the configured priority value requires a switchover, the system starts a 60-second timer before performing a switchover. After the switchover, the highest priority standby controller becomes the active controller.
  • During a stack merge, the active controller with the larger number of units wins the election and reloads every unit of the losing stack.
  • If two stacks have the same number of units, the active controller election criteria are based, in order, on priority, system up time, ID, and finally the MAC address.
  • If two stacks have the same number of units and the same priority, then the stack with the longest system up time (by 30 seconds or more) wins the election. Otherwise, the smallest ID is compared next, followed by MAC address. If the losing active controller has the highest priority, it becomes the standby controller after reloading and relearning the protocols. It becomes the active controller after an internal switchover.
Note: If the active controllers of two merging stacks have different hitless stacking settings (for example, hitless stacking is enabled in one stack and disabled in the other), the winning controller retains its hitless stacking setting and runtime configuration for the merged stack.

You can use the show stack command to check whether hitless stacking is enabled.

  • If hitless-failover enable is configured, the following is displayed: 
    Standby u2 - protocols ready, can failover or manually switch over
  • If hitless-failover enable is not configured, the following is displayed: 
     Standby u2 - No hitless failover. Reason: hitless-failover not configured

Note: Whether or not hitless-failover is enabled, if the entire stack reloads, and the active controller does not come up, the bootup standby takes over in four minutes or longer, depending on the size of the startup-config file, to allow the active controller sufficient time to process a very large startup-config file during bootup. The bootup standby takes over by reloading every unit including itself. A bootup standby is the device that was the standby controller before the reload. It may not be the current standby controller.

Parent topic: Hitless Stacking