Campus lan and Wireless lan solution Design Guide

In-Service Software Upgrades (ISSUs) 
In-Service Software Upgrade (ISSU) is a process that upgrades an image to another image on a device while 
the network continues to forward packets. ISSU helps network administrators avoid a network outage when 
performing a software upgrade. The images are upgraded in install mode wherein each package is upgraded 
individually. ISSU is supported in Catalyst 9000 Series standalone and modular platforms (Catalyst 9400, 9500, 
and 9600 Series). 
Note: 
ISSU is not supported for an upgrade from Cisco IOS XE Fuji 16.9.1 to Cisco IOS XE Fuji 16.9.2. ISSU 
from Cisco IOS XE Fuji 16.9.x to Cisco IOS XE Gibraltar 16.10.x or Cisco IOS XE Gibraltar 16.11.x is not 
supported. On Cisco Catalyst 9500 Series Switches - High Performance, ISSU with Cisco StackWise Virtual is 
supported starting from Cisco IOS XE Gibraltar 16.12.1. Therefore, ISSU upgrades can be performed only 
starting from this release to a later release. 
Graceful Insertion and Removal (GIR) 
GIR leverages redundant paths and existing routing protocols to gracefully isolate a device without impacting 
active flows. Conversely, GIR also gracefully reinserts the device back into service when the work is complete. 
GIR allows the network administrator to easily manipulate the routing and first-hop gateway metrics of a 
network device that is about to undergo maintenance to make it a very unattractive path. It does this by inflating 
metrics or sending messages to indicate to peers that this device is no longer the best path for traffic. Once the 
traffic moves away from the device, maintenance actions can be undertaken. Once the maintenance is 
complete, returning these metrics to their former values then smoothly restores normal traffic flow. 
Fast Software Upgrade (FSU) and Extended Fast Software Upgrade 
During a software upgrade on the switch, user traffic is disrupted until the new software completely boots up. 
The traffic downtime is a concern for customers running critical applications. The Fast Software Upgrade (FSU) 
feature significantly reduces the traffic downtime during a software upgrade. The fast software upgrade feature 
is supported on both stacking and standalone systems from IOS XE 16.8.1a and higher. 

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Note: 
Fast software upgrade is supported only on access switches with a single logical uplink connection. Fast 
software upgrade is not supported if the Micro Controller Unit (MCU) Field Programmable Gate Array (FPGA) 
upgrade is required. Fast software upgrade is not supported if the switch is configured as StackWise Virtual 
System. 
Extended Fast Software Upgrade reduces the traffic downtime during software reload or upgrade operations. 
Compared to Fast Software Upgrade, the traffic downtime is reduced to less than 30 seconds, depending on 
the switch configuration. Extended Fast Software Upgrade uses graceful restart capability (a feature of Cisco 
NSF) to ensure that device configurations, such as certain routing protocols, remain unaffected during a 
software upgrade or reload. 
The following table summarizes high availability support with the various Catalyst 9000 Series switches.