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# Before You Begin a Unified In-Service Software Upgrade

The following hardware and software prerequisites must be met for the successful completion of unified ISSU. You can issue the show issu command to determine whether the routers meets these requirements.

## Hardware Requirements for Unified ISSU

- The router must support unified ISSU. Therefore the router must be an E120, E320, or ERX1440 router.
- Two SRP modules must be installed in the router.
- All installed combinations of line modules and IOAs must support unified ISSU. Unsupported modules that are online are reloaded during the unified ISSU, with consequent loss of connections and traffic forwarding.

  Do not install IOAs in the chassis while the unified ISSU operation is in process.

- The redundant SRP module must have at least 300 MB of free memory. Depending on their configuration, line modules require up to 75 MB of free memory.

For information about modules supported on E120 and E320 routers:

- See E120 and E320 Module Guide, Table 1, Modules and IOAs for detailed module specifications.
- See E120 and E320 Module Guide, Appendix A, IOA Protocol Support for information about the modules that support unified ISSU.
- On the ERX1440, certain hardware updates might require a module to be cold restarted. Unified ISSU cannot be successfully accomplished with such modules. In this case, the behavior is the same as for unsupported line modules. The unified ISSU process reboots these modules and holds them down until the supported modules on the router complete the unified ISSU process.

  When hardware updates are required for modules that you have installed in an ERX1440 router, contact your Juniper Networks representative to determine whether the update affects unified ISSU.

## Software Requirements for Unified ISSU

- The running JUNOSe software release must support unified ISSU.

  You can upgrade to a software version that supports unified ISSU from a software version that does not support unified ISSU only by means of a conventional upgrade. During the conventional upgrade, all line modules are reloaded, all subscribers are dropped, and traffic forwarding is interrupted until the upgrade is completed.

- The armed (upgrade) release must be capable of being upgraded to from the currently running release; it must be higher-numbered than the running release.
- All applications that are configured on the router must support unified ISSU and stateful SRP switchover.

  If one or more unified ISSU-challenged applications are configured and you proceed with a unified in-service software upgrade, the unified ISSU process forces a conventional upgrade on the router. All line modules are reloaded, all subscribers are dropped, and traffic forwarding is interrupted until the upgrade is completed.

  You can avoid this circumstance by removing the configuration for the unified ISSU-challenged applications from the router before you begin the in-service software upgrade.

  See Application Support for Unified ISSU for information about whether an application supports unified ISSU.

- Stateful SRP switchover must be configured on the router. Use the following commands to configure high availability:
  `
  host1(config)#redundancy
  host1(config-redundancy)#mode high-availability
  `

  See Managing Modules for information about high availability.

The following requirements must be met for traffic forwarding to continue. However, failing to meet these requirements does not halt the unified ISSU operation. The unified ISSU process offers the option to override or ignore these forwarding requirements.

- Graceful restart must be enabled for all configured routing protocols. The unified ISSU operation relies on graceful restart to keep the routing protocols alive through the various stages of the upgrade.
- All connected peers must be configured with graceful restart. Because some protocols cannot themselves confirm peer configuration for graceful restart, you must ensure that the peers are properly configured.
- For applications that exchange keepalive messages with peers, you must ensure that the poll times are adequate to maintain the peering session across any forwarding interruption caused by the unified ISSU operation.
- On the ERX1440, you must enter the key provided with your license in order to make the unified ISSU CLI commands available. Unified ISSU is licensed on only the ERX1440 router; no license is required or available on the E120 and E320 routers.

  The license issu command is available only on the ERX1440 CLI.

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