# Benefits of Stateful Line Module Switchover

Line module high availability enhances the overall reliability and resiliency of the router. All subscribers who are connected during line module recovery remain active; their sessions are preserved during switchover and forwarding of data through the fabric slice is disrupted briefly. When a switchover occurs with the high availability state being active, a message is displayed on the active SRP module after the secondary line module has successfully taken over from the previously configured primary line module.

If you configured the ha logging event category (using the log severity notice ha command), log messages are displayed when high availability for line modules is enabled or disabled, either because of manual settings or in response to system events. For example, the following log message is displayed when a line module event causes high availability to be disabled.

NOTICE 01/27/2004 19:54:06 ha (): High Availability disabled due to secondary line card in slot 9 down

The commands used to configure stateful switchover for SRP modules and line modules are very similar. The configuration modes are the same. The keywords to configure stateful switchover for SRP modules and for line modules are different. When redundancy mode is configured for high availability on E120 and E320 routers with dual SRP modules, high availability is enabled only for the corresponding SRP modules. You must explicitly specify high availability for the line modules to enable stateful switchover of the line modules to occur.

## 1:1 Redundancy Model

Line module high availability uses a 1:1 redundancy model to maintain subscriber sessions, during and after a switchover of the primary line module to the secondary line module. This feature is supported only on E120 and E320 routers installed with ES2 4G LMs and Service IOAs. You can enable line module high availability feature only on the compatible line modules and IOA combinations. On routers in which line module high availability is disabled or not available for configuration (cold-restart support is not present), all subscriber sessions are forcibly terminated when a line module fails or stops responding. This mechanism is known as a cold-restart.

## Seamless Preservation of Subscriber Sessions

When the secondary line module in the HA configuration becomes the primary line module, all the applications on it recover to a stable state to operate normally in the place of the failed line module. The new primary line module maintains existing subscriber sessions and continues to forward subscriber data traffic after the switchover. A brief subscriber data disconnection occurs for two minutes during the switchover. Diagnostic functions are still run on the failed line module to ensure that the hardware on the defective line module is still usable. The exception might have been triggered by a hardware fault that the diagnostic services might discover using its testing mechanism. For the stateful line module switchover to work correctly, the current subscriber information must be made accessible to the newly configured primary line module. The states of subscriber sessions that were on the failed line module are mirrored to the secondary line module, which has taken over as the primary controller.

Only those subscriber sessions whose operational status is up are guaranteed to be retained. Other subscriber connections that are in the process of fluctuating between up and down states might not be preserved. Such a technique of not retaining the subscriber sessions that are constantly alternating is adopted because of the possibility of occurrence of faults during transitional periods of states. If subscriber sessions in these transitional states are not preserved, the possibility of the same problem occurring on the secondary line module after the switchover is reduced.

#### Related Documentation

- Stateful Line Module Switchover Overview
- Stateful Line Module Switchover Platform Considerations
- System Operations When Stateful Line Module Switchover Is Enabled