# Understanding SRP Module Redundancy

| Note: This section does not cover NVS cards or the behavior on systems running high availability features such as hitless SRP switchover. For information about managing NVS in a router that contains two SRP modules, see Managing Flash Cards on SRP Modules in the JunosE System Basics Configuration Guide. For information about managing high availability in a router, see Managing Stateful SRP Switchover.

The SRP module uses a 1:1 redundancy scheme. When two SRP modules are installed in the router, one acts as a primary and the second as a redundant module. On ERX7xx models, ERX14xx models, and the ERX310 router, both SRP modules share a single SRP I/O module located in the rear of the chassis. On the E120 and E320 routers, both SRP modules share an SRP IOA located in the rear of the chassis.

| Note: The ERX310 router does not support SRP module redundancy. For this reason, any references to synchronization that may appear in command output or system messages do not apply to the ERX310 router.

After you install two SRP modules, the modules negotiate for the primary role. A number of factors determine which module becomes the primary; however, preference is given to the module in the lower-numbered slot. The SRP modules record their latest roles and retain them the next time you switch on the router.

With the default software settings, if the primary SRP module fails, the redundant SRP module takes control without rebooting itself. For information about preventing the redundant SRP module from taking control, see Managing SRP Module Redundancy.

On E120 and E320 routers, the switch fabric is distributed between the SFMs and the SRP modules. If the primary SRP module fails a diagnostic test on its resident slice of switch fabric, then it gives back control to the redundant SRP module if both of the following are true:

- The standby SRP module does not indicate any error.
- The standby SRP module passes diagnostics on its attached fabric slice.

When the redundant SRP module takes control, the following sequence of events occurs:

1. The original primary SRP module reboots and takes the redundant role.
2. The redundant SRP module restarts and takes the primary role without reloading new code. (When upgrading software, you must reload the software on the redundant SRP module. See Installing JunosE Software in the JunosE System Basics Configuration Guide.)
3. All line modules reboot.

The following actions activate the redundant SRP module:

- Failure of the primary SRP module (hardware or software)
- Pushing the recessed reset button on the primary SRP module. (See Figure 2 and Figure 3.)
- Issuing the srp switch command
- Issuing the redundancy force-switchover command

  Figure 2: SRP Module on ERX7xx Models and ERX14xx Models

  Figure 3: SRP Module on the E120 and E320 Routers

#### Related Documentation

- Understanding Configuration of SRP Modules for Redundancy
- Installing a Redundant SRP Module
- Managing SRP Module Redundancy