Redundancy group switchover

Redundancy group switchovers include automatic switchovers and manual switchovers. The hold down timer and the preemption timer are used to control automatic switchovers.

Timers for automatic switchovers include the hold down timer and the preemption delay timer.

• Hold down timer—The hold down timer specifies the minimum interval between two switchovers to prevent frequent switchovers. The timer starts when a switchover is finished. The redundancy group can perform another switchover only after the hold down timer expires.

• Preemption delay timer—The preemption delay timer specifies the delay for a switchover to the high-priority node. The preemption delay timer starts when the switchover is triggered. The redundancy group performs the switchover only after the timer expires. The delay allows the system to process events (such as interface state changes) required for the switchover.

Typically, a redundancy group uses the high-priority node to forward traffic. This requires the redundancy group to perform switchovers automatically.

• When the high-priority node fails, the redundancy group switches over to the low-priority node.

• When the high-priority node recovers, the redundancy group switches over to the high-priority node.

A redundancy group cooperates with the Track module to monitor link and interface status for its nodes and perform automatic switchovers.

A redundancy group node has a weight of 255 (not configurable). Each redundancy group node is associated with one or multiple track entries that have a user-configurable weight decrement rate. When the state of a track entry changes, the weight of the associated node is reduced or increased by the weight decrement rate of the track entry.

• When the state of the track entry changes to NotReady or Negative, the node weight is reduced by the weight decrement rate.

• When the state of the track entry changes to Positive, the node weight is increased by the weight decrement rate.

	NOTE: For a track entry to operate correctly, you must configure the track entry as described in "Configuring Track."

When the node weight is lower than or equal to 0, the redundancy group node is considered as failed. This triggers a switchover to the other redundancy group node. Typically, the switchover is from the high-priority node to the low-priority node.

The Reth module shuts down all interfaces on the failed high-priority node. The shutdown interfaces cannot recover automatically. When you associate a track entry with the high-priority node, you must exclude the tracked interface from the shutdown action if the interface has one of the following roles:

• Member of the redundancy group.

• Member of a Reth interface in the redundancy group.

Excluded interfaces can recover automatically. This changes the state of their track entries to Positive. The redundancy group switches over to the high-priority node when the following requirements are met:

• The weight of the high-priority node is increased to a positive number.

• The track entry state changes to Positive for the first excluded interface that goes down.

You can perform manual switchovers in one of the following situations:

• Automatic switchover is disabled.

  Automatic switchover is disabled for a redundancy group in one of the following situations:

  • No tracked interfaces are excluded from the shut down action by the Reth module when the interfaces have one of the following roles:

    • Member of the redundancy group.

    • Member of a Reth interface in the redundancy group.

  • The preemption delay timer is set to 0.

• Switchovers are required when both redundancy group nodes operate correctly. For example, component replacement is required on the high-priority node.

Manual switchovers are triggered by commands:

• The switchover request command issues a request for a switchover to the low-priority node.

• The switchover reset command issues a request for a switchover to the high-priority node.