John,
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. This type of switchover ensures that Layer 2 traffic is not interrupted.
In networking devices running SSO, both supervisor engines must be running the same configuration 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. The switch requires between 0 and 3 seconds to switchover from the active to the redundant supervisor engine.
NSF Operation
Cisco NSF always runs with SSO and provides redundancy for Layer 3 traffic. NSF works with SSO to minimize the amount of time that a network is unavailable to its users following a switchover. The main purpose of NSF is to continue forwarding IP packets following a supervisor engine switchover.
Cisco NSF is supported by the BGP, OSPF, EIGRP, and IS-IS protocols for routing and is supported by Cisco Express Forwarding (CEF) for forwarding. The routing protocols have been enhanced with NSF-capability and awareness, which means that routers running these protocols can detect a switchover and take the necessary actions to continue forwarding network traffic and to recover route information from the peer devices. The IS-IS protocol can be configured to use state information that has been synchronized between the active and the redundant supervisor engine to recover route information following a switchover instead of information received from peer devices.
A networking device is NSF-aware if it is running NSF-compatible software. A device is NSF-capable if it has been configured to support NSF; it will rebuild routing information from NSF-aware or NSF-capable neighbors.
Each protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) tables. After the routing protocols have converged, CEF updates the FIB table and removes stale route entries. CEF then updates the modules with the new FIB information.
https://supportforums.cisco.com/docs/DOC-4028
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/nsfsso.html
http://www.networkers-online.com/blog/2008/11/cisco-and-nonstop-forwarding/
HTH
REgards
Inayath
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