Nsf Operation; Cisco Express Forwarding - Cisco 7604 Configuration Manual

Chapter 7 Configuring NSF with SSO Supervisor Engine Redundancy
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, 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 line cards with the new
FIB information.

Cisco Express Forwarding

A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided
by Cisco Express Forwarding (CEF). CEF maintains the FIB, and uses the FIB information that was
current at the time of the switchover to continue forwarding packets during a switchover. This feature
reduces traffic interruption during the switchover.
During normal NSF operation, CEF on the active supervisor engine synchronizes its current FIB and
adjacency databases with the FIB and adjacency databases on the redundant supervisor engine. Upon
switchover of the active supervisor engine, the redundant supervisor engine initially has FIB and
adjacency databases that are mirror images of those that were current on the active supervisor engine.
For platforms with intelligent line cards, the line cards will maintain the current forwarding information
over a switchover. For platforms with forwarding engines, CEF will keep the forwarding engine on the
redundant supervisor engine current with changes that are sent to it by CEF on the active supervisor
engine. The line cards or forwarding engines will be able to continue forwarding after a switchover as
soon as the interfaces and a data path are available.
As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates will cause
prefix-by-prefix updates to CEF, which it uses to update the FIB and adjacency databases. Existing and
new entries will receive the new version ("epoch") number, indicating that they have been refreshed. The
forwarding information is updated on the line cards or forwarding engine during convergence. The
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Understanding NSF with SSO Supervisor Engine Redundancy
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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