Router Behavior During a Unified In-Service Software Upgrade

The following behaviors are characteristic of a unified in-service software upgrade.

Connections that were established before you begin the in-service software upgrade are maintained across the upgrade. Any such connection that was forwarding data continues to do so during and after the upgrade.
New connections are denied until the upgrade is completed.
Packet loss during the upgrade is limited. Bandwidth through the modules is reduced, but the impact is minimal.
Graceful restart protocols do not time out during the in-service software upgrade.
The unified in-service software upgrade has a minimal effect on the control and data planes. During the SRP module upgrade phase, forwarding through the fabric is interrupted for about 1 second on the E120 and E320 routers and about 4 seconds on the ERX-1440 router. During the line module upgrade phase, forwarding through the chassis is interrupted for about 15 seconds on the E120 and E320 routers and for about 50 seconds on the ERX-1440 router.
Diagnostic software is not run on any modules during a unified in-service software upgrade.
The router will undergo a cold restart if you attempt to upgrade the software to a lower-numbered version with unified ISSU. The in-service software upgrade must be to a higher-numbered release than the running release.
Additional memory is consumed during a unified in-service software upgrade. Available memory on a line module might not be sufficient due to the module’s configuration. Unified ISSU can detect this limitation during the upgrade procedure and exit the process.