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.
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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.
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