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# Redundancy Modes of Operation

The switch route processor (SRP) modules can operate in one of two redundancy modes—file system synchronization and high availability.

## File System Synchronization Mode

File system synchronization is the default behavior mode for E-series routers that contain redundant SRPs. Available only to SRP modules, this mode has been available since the 2.x release. In this mode:

- Files and data (for example, configuration files and releases) in nonvolatile storage (NVS) remain synchronized between the primary and standby SRP modules.
- SRP modules will reload all line modules and restart from saved configuration files.
- If the active SRP module switches over to the standby SRP, the router cold-restarts as follows:
  - All line modules are reloaded.
  - User connections are lost, and forwarding through the chassis stops until the router SRP module recovers.
  - The standby SRP module boots from the last known good configuration from NVS.

For additional information about the default SRP functionality, see Managing Modules.

## High Availability Mode

Currently applicable to the SRP module, the Juniper Networks high availability mode uses an initial bulk file transfer and subsequent, transaction-based mirroring to ensure rapid SRP module recovery after a switchover. This process is referred to in this chapter as stateful SRP switchover .

In addition to keeping the contents of NVS, high availability mode keeps state and dynamic configuration data from the SRP memory synchronized between the primary and standby SRP modules.

When stateful SRP switchover is enabled, an SRP switchover keeps line modules up and forwarding data, and the newly active SRP module continues from the point of switchover.

By using transaction-based mirroring instead of file synchronization, high availability mode keeps the standby SRP module synchronized with the active SRP module. Mirroring occurs from memory on the active SRP module to memory on the standby SRP module by way of transactions. When a transaction is committed on the active SRP module, the data associated with the transaction is sent to the standby SRP module.

In high availability mode:

- The contents of the NVS in the primary and standby SRP modules remain synchronized.

  | Note: Configuration files are always synchronized. Nonconfiguration files are synchronized when the disable-autosync command has not been configured; this is the default case. When the disable-autosync command has been configured, nonconfiguration files are not synchronized.

- If a switchover occurs:
  - The standby SRP 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 (or after 5 minutes).

      | Note: If the switchover does not finish within 5 minutes, the SRP module cancels the operation and reenables CLI configuration.

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