Implementing Hardware and Software Redundancy on Modular Switches
What is Nonstop Forwarding – NSF?

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 and the subsequent establishment of the routing protocols peering relationships.

Cisco NSF is supported by the BGP, OSPF, IS-IS and EIGRP 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 remote routers running these protocols and configured for NSF can detect a switchover, recover route information from the peer devices and take the necessary actions to continue forwarding network traffic. 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 rather than using 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 line cards with the new FIB information.

Cisco NSF provides these benefits:

  • Improved network availability
  • Network stability may be improved with the reduction in the number of route flaps
  • Because the interfaces remain up throughout a switchover, neighboring routers do not detect a link flap (the link does not go down and come back up).
  • User sessions established before the switchover are maintained.
CAUTION:

There are a number of caveats and restrictions which apply to this process. Consult the Cisco documentation for specific information for your particular hardware and software combination.