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Software Configuration Guide—Release 15.0(2)SG
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Chapter 9 Configuring Cisco NSF with SSO Supervisor Engine Redundancy
About NSF with SSO Supervisor Engine Redundancy
NSF with SSO Supervisor Engine Redundancy Overview
Catalyst 4500 series switches support fault resistance by allowing a redundant supervisor engine to take
over if the primary supervisor engine fails. NSF works with SSO to minimize the amount of time a
network is unavailable to its users following a switchover.
NSF provides these benefits:
• Improved network availability
NSF continues forwarding network traffic and application state information so that user session
information is maintained after a switchover.
• Overall network stability
Network stability may be improved with the reduction in the number of route flaps, which were
created when routers in the network failed and lost their routing tables.
• Neighboring routers do not detect a link flap
Because the interfaces remain up during a switchover, neighboring routers do not detect a link flap
(the link does not go down and come back up).
• Prevents routing flaps
Because SSO continues forwarding network traffic during a switchover, routing flaps are avoided.
• Maintains user sessions established prior to the switchover
Catalyst 4500 series switches also support route processor redundancy (RPR). For information about
these redundancy modes, see Chapter 8, “Configuring Supervisor Engine Redundancy Using RPR and
SSO.”
SSO Operation
SSO establishes one of the supervisor engines as active while the other supervisor engine is designated
as standby, and then SSO synchronizes information between them. A switchover from the active to the
redundant supervisor engine occurs when the active supervisor engine fails, or is removed from the
switch, or is manually shut down for maintenance.
In networking devices running SSO, both supervisor engines must be running the same Cisco IOS
software version and ROMMON version so that the redundant supervisor engine is always ready to
assume control following a fault on the active supervisor engine. SSO switchover also preserves FIB and
adjacency entries and can forward Layer 3 traffic after a switchover. Configuration information and data
structures are synchronized from the active to the redundant supervisor engine at startup and whenever
changes to the active supervisor engine configuration occur. Following an initial synchronization
between the two supervisor engines, SSO maintains state information between them, including
forwarding information.
During switchover, system control and routing protocol execution is transferred from the active
supervisor engine to the redundant supervisor engine.
Note Use the [no] service slave-log configuration command to forward all error messages from the standby
supervisor engine to the active engine. By default, this capability is enabled. For details, refer to the
Catalyst 4500 Series Switch Cisco IOS System Error Message Guide, Release 12.2(37)SG.
