Cisco In-Service Software Upgrade (ISSU) allows a customer to upgrade or downgrade complete IOS images with minimal to no impact on a Catalyst 4500 system with redundant supervisor engines. It enables rapid, non-disruptive software upgrades for new line cards, new power supplies, new features, or bug fixes. ISSU offers continuous packet forwarding during the supervisor engine switchover running different IOS software versions. Together with Nonstop Forwarding/Stateful Switchover (NSF/SSO), ISSU allows forwarding of data packets along known routes without any route flaps or network instabilities during the software upgrade. As a result, IP phone calls do not drop even when the IOS Software images are upgraded or downgraded. There is no impact to the overall network capacity since the switch and all the forwarding paths remain active during the entire upgrade process. ISSU is typically deployed in the enterprise wiring closet or service provider Metro Ethernet aggregation. ISSU eliminates planned downtime to maximize system and network availability.