What Is an In-service Software Upgrade?
An in-service software upgrade (ISSU) allows stack units or units in a Campus Fabric
(SPX) system to be upgraded with minimal interruptions to multi-unit topologies.
ISSU provides an incremental method to upgrade traditional stacks and Campus Fabric (SPX) systems. 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.
Note: Traffic through PE nodes is affected by ISSU because PE ports cannot be part of a
traditional LAG.