In-Service Software Upgrade Overview
An in-service software upgrade (ISSU)
allows stack units to be upgraded with minimal interruptions to multi-unit topologies.
ISSU provides an incremental method to upgrade traditional stacks. A successful ISSU does not affect uplink or downlink connectivity in a topology with multi-unit LAGs. Only the node that is undergoing the upgrade requires a hardware reset that includes the reset of the packet processor. As a result, traffic transiting only that node is disrupted.