Stateful Switchover Overview - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 7 Configuring the Cisco IOS In-Service Software Upgrade Process
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Stateful Switchover Overview

Development of the SSO feature is an incremental step within an overall program to improve the
availability of networks constructed with Cisco IOS 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 processor 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 7-1
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 to restore network capacity and service for certain failures, which
leads to additional availability.
Changeversion Process, page 7-12
Guidelines for Performing ISSU, page 7-13
Versioning Capability in Cisco IOS Software to Support ISSU, page 7-13
SNMP Support for ISSU, page 7-15
Compatibility Verification Using Cisco Feature Navigator, page 7-15
illustrates how SSO is typically deployed in service provider networks. In this example,
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.0E and IOS 15.2(5)E
About ISSU
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