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.
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.
Figure 7. Cisco NSF with
SSO Network Deployment: Service Provider Networks
of availability may be gained by deploying Cisco NSF with SSO at other points
in the network where a single point of failure exists. Figure below illustrates
an optional deployment strategy that applies Cisco NSF with SSO at the
enterprise network access layer. In this example, each access point in the
enterprise network represents another single point of failure in the network
design. In the event of a switchover or a planned software upgrade, enterprise
customer sessions would continue uninterrupted through the network.
Figure 8. Cisco NSF with
SSO Network Deployment: Enterprise Networks
information on SSO, see the Stateful Switchover document.