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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 8 Configuring the Cisco IOS XE In Service Software Upgrade Process
About Performing ISSU
Stateful Switchover
Development of the SSO feature is an incremental step within an overall program to improve the
availability of networks constructed with Cisco IOS XE switches.
In specific Cisco networking devices that support dual supervisor engines, SSO takes advantage of
supervisor engine redundancy to increase network availability. SSO achieves this by establishing one of
the supervisor engines as the active processor while the other supervisor engine is designated as the
standby processor. Following an initial synchronization between the two supervisor engines, SSO
dynamically synchronizes supervisor engine state information between them in real-time.
A switchover from the active to the standby supervisor engine occurs when the active supervisor engine
fails or is removed from the networking device.
Cisco NSF is used with SSO. Cisco NSF allows the forwarding of data packets to continue along known
routes while the routing protocol information is being restored following a switchover. With Cisco NSF,
peer networking devices do not experience routing flaps, which reduce loss of service outages for
customers.
Figure 8-1 illustrates how SSO is typically deployed in service provider networks. In this example,
Cisco NSF with SSO is enabled at the access layer (edge) of the service provider network. A fault at this
point could result in loss of service for enterprise customers requiring access to the service provider
network.
For Cisco NSF protocols that require neighboring devices to participate in Cisco NSF, Cisco NSF-aware
software images must be installed on those neighboring distribution layer devices. Depending on your
objectives, you may decide to deploy Cisco NSF and SSO features at the core layer of your network.
Doing this can help reduce the time required to restore network capacity and service for certain failures,
which leads to additional availability.
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