CEF/NSF/SSO

Supervisor engine redundancy using Cisco NSF with SSO

This would be another one of those areas where I find myself googling, "Difference between NSF and CEF" or "Difference between FIB and RIB."  Today, I try to clearify this so that a 5 year old could understand it!

Starting off, Non Stop Forwarding (NSF) is the capacity of a device to forward packets when a supervisor engine switchover happens.  Basically, its to keep the routing protocols from reconverging when a single supervisor module fails by continuing to forward IP packets.  NSF works with Stateful Switchover (SSO) to minimize the time that a layer 3 network is unavailable.   NSF is supported by EIGRP, OSPF, BGP, and IS-IS.  A router running these protocols will continue forwarding traffic once an internal switchover is detected.  

Note:  For NSF to operate, a device's neighbors MUST BE NSF AWARE!

SSO maintains two seperate supervisor modules, with one designated as the as the active and the other as the standby.  SSO synchronizes configuration information between the seperate supervisor modules so that system control and routing protocol execution is transferred from the active to the standby supervisor engine.  SSO allows the standby RP (route processor) to take control of the device after a hardware or software fault on the active RP.

Note:  Both supervisor engines MUST have the same Cisco IOS.

Cisco Express Forwarding (CEF) is the mechanism that is used to ensure stability between the Forwarding Information Base (FIB) and the Routing Information Base (RIB).  CEF on the active supervisor engine synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the redundant supervisor engine.  Each routing protocol is dependent on CEF to continue forwarding packets during switchover while the routing protocols rebuild the RIB tables.  Once reconverged, CEF updates the FIB. 

Wait..what?  What is FIB/RIB?

Forwarding Information Base (FIB) and Cisco Express Forwarding (CEF) switching were introduced to make layer-3 switching deterministic. When IP routes are copied from RIB to FIB, their next hops are resolved, outgoing interfaces are computed and multiple entries are created when the next-hop resolution results in multiple paths to the same destination.  As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates cause prefix-by-prefix updates to CEF, which is used to update the FIB and adjacnesy databases.  At this time, stale route entries in the FIB are removed. 

Note: 
Cisco NSF & SSO have the most impact in the access layer, as an access layer failure is a single point of failure that causes outages for the end devices connected to it. 

NSF works with the SSO to continue forwarding IP packets following an RP fail-over.