Difference between NSF and SSO

[JohnTylerPearce]

I've been doing some research on these technologies, since they are one of my weakpoints.

But, what are the differences between the two and how do they relate? I don't need a 5 paragraph

response, but if you want too, feel free. I'm all about doing my own research, but, I just needed

some help understanding how they interact.

[InayathUlla Sharieff · Accepted solution]

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

*Plz rate if this info is helpfull.

[mfurnival]

The fundamentals of this are:

SSO - switch synchonizes certain features across the active and redundant supervisors which means it can switch over seamlessly in the event of a failure (more concerned with layer 2 forwarding).

NSF with SSO - switch can continue to forward data along existing routes while routing reconvergences (more concerned with layer 3).

The paragraphs on this page are simple and concise explanations:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd801c5cd7.html

[JohnTylerPearce]

Thanks for all your responses, I really appreciate it.

[Joseph W. Doherty]

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Posting

John, the other posts summarize SSO vs. NSF well, but I want to emphasis a point about NSF.  Unlike SSO, NSF also tries to deal with routing neighbors/peers.  During a switch over, if routing neighbors/peers see a device "drop" they will route around it (if they can).  So, not only must the NSF device be able to continue to forward its transit traffic, it doesn't want its neighbors/peers to see it as lost/off-line.

I think I also recall NSF has special features between routing neighbors/peers that neighbors/peers have a "backdoor" to share some L3 info without impacting the rest of the L3 routing topology.  (I.e. I think I recall NSF "aware" devices don't support NSF for themselves, but they help support neighbors/peers that are fully NSF capable.)