When you are planning to perform a unified
in-service software upgrade (unified ISSU), choose a time when your
network is as stable as possible. As with a normal upgrade, Telnet
sessions, SNMP, and CLI access are briefly interrupted. In addition,
the following restrictions apply:
The master Routing Engine and backup Routing Engine must
be running the same software version before you can perform a unified
ISSU.
During a unified ISSU, you cannot bring any PICs online
or offline.
