protection for network edge devices with dual RPs that represent a single point
of failure in the network design, and where an outage might result in loss of
service for customers.
In Cisco networking
devices that support dual RPs, SSO takes advantage of RP redundancy to increase
network availability. The feature establishes 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.
SSO is used with the
Cisco Nonstop Forwarding (NSF) feature. 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.
The figure below
illustrates how SSO is typically deployed in service provider networks. In this
example, Cisco NSF with SSO is primarily 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.
Figure 1. Cisco NSF with SSO Network
Deployment: Service Provider Networks
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. Additional network availability benefits might be achieved by
applying Cisco NSF and SSO features at the core layer of your network; however,
consult your network design engineers to evaluate your specific site
Additional levels of
availability may be gained by deploying Cisco NSF with SSO at other points in
the network where a single point of failure exists. The 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 2. Cisco NSF with SSO Network
Deployment: Enterprise Networks