Stateful Line Module Switchover Modes

The line modules can operate in one of two redundancy modes—stateless switchover and high availability.

Stateless Switchover Mode

Stateless switchover is the default behavior mode for E120 and E320 routers that contain redundant line modules. This mode is available on line modules that you configured to function in a redundancy group and on which the high availability behavior is not enabled using the mode high-availability slot command. During switchover, the line, circuit, and IP interfaces on one or more IOAs appear to go down temporarily. The duration of the downtime depends on the number of interfaces and the size of the routing table, because the router must reload the interface configuration and the routing table from the SRP module.

High Availability Mode

The high availability mode uses an initial bulk file transfer and subsequent, transaction-based mirroring to ensure rapid line module recovery after a switchover. This process is referred to as stateful line module switchover.

High availability mode keeps state and dynamic configuration data from the SRP memory synchronized between the primary and standby line modules.

When stateful line module switchover is enabled, user sessions are unaffected and forwarding of data is briefly disrupted, and the newly active line module continues from the point of switchover.

In high availability mode:

• The contents of the volatile storage in the primary and standby line modules remain synchronized using the mirroring methodology.

If a switchover occurs:

• The standby line module warm-restarts using the mirrored data to restore itself to the state of the system before the switchover.
• During the warm restart:
  • User connections remain active, and forwarding continues through the chassis.
  • New user connection attempts during switchover are denied until switchover is complete.
  • New configuration changes are prevented until switchover is complete.