Manual

Cisco IOS In Service Software Upgrade Process
Information About Performing ISSU
10
Cisco IOS Release 12.2(28)SB
Figure 6 ISSU States During the ISSU Process
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 routers.
In specific Cisco networking devices that support dual RPs, SSO takes advantage of RP redundancy to
increase network availability by establishing one of the RPs as the active processor while the other RP
is designated as the standby processor, and then synchronizing critical state information between them.
Following an initial synchronization between the two processors, SSO dynamically maintains RP state
information between them.
A switchover from the active to the standby processor occurs when the active RP fails, is removed from
the networking device, or is manually taken down for maintenance.
Cisco NSF is used with SSO. Cisco NSF allows for 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, thereby reducing loss of service
outages for customers.
Figure 7 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 to restore network capacity and service for certain failures, which
leads to additional availability.
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