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JUNOSe 11.1.x Service Availability Configuration Guide
Unified ISSU Overview
In software releases numbered lower than Release 6.0, all line modules are reloaded
when an SRP switchover occurs. This reload disconnects user sessions and disrupts
forwarding through the chassis. Stateful SRP switchover was introduced in JUNOSe
Release 6.0 to minimize the impact to the router of a stateful switchover from the
active SRP module to the standby SRP module. Stateful SRP switchover (high
availability) maintains user sessions during the switchover and data forwarding
through the router continues to flow with little impact, thus improving the overall
availability of the router.
The unified in-service software upgrade (unified ISSU) feature further extends router
availability. Unified ISSU enables you to upgrade the router to a higher-numbered
software release without disconnecting user sessions or disrupting forwarding through
the chassis.
A conventional software upgrade one that does not use the unified ISSU
process causes a router-wide outage for all users. Only static configurations (stored
on the flash card) are maintained across the upgrade; all dynamic configurations are
lost. A conventional upgrade takes 30-40 minutes to complete, with additional time
required to bring all users back online.
When you perform a unified in-service software upgrade on a router that has one or
more modules that do not support unified ISSU, these modules alone are upgraded
by means of the legacy, conventional upgrade process. The unsupported modules
undergo a cold reboot at the beginning of the unified ISSU process, and are held
down until the in-service software upgrade is completed. Connections that pass
through the unsupported modules are lost. The interfaces on these modules pass
into a down state, which causes the physical layer and link layer to go down during
the unified in-service software upgrade for those modules.
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Unified ISSU Overview
Unexpected Suspension of Subscriber Login and Logouts During Unified
ISSU on page 90
Unexpected SONET and SDH Behavior During Unified ISSU on page 91
Unexpected T3 Behavior During Unified ISSU on page 91
Unavailability of TACACS+ Services During Unified ISSU on page 92
Interruption in Traffic Forwarding for Layer 3 Routing Protocols During Unified
ISSU on page 92
Recommended Settings for Routing Protocol Timers During Unified
ISSU on page 94
Upgrading Router Software with Unified ISSU on page 96
Halt of Unified ISSU During Initialization Phase Overview on page 99
Halting Unified ISSU During Initialization Phase on page 99
Halt of Unified ISSU During Upgrade Phase Overview on page 100
Halting Unified ISSU During Upgrade Phase on page 100
Monitoring the Status of the Router During Unified ISSU on page 101
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