Cisco Express Forwarding - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 13 Configuring Cisco NSF with SSO Supervisor Engine Redundancy

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 forwarding engines, CEF keeps the forwarding engine on the redundant supervisor
engine current with changes that are sent to it by CEF on the active supervisor engine. The forwarding
engine can 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 cause
prefix-by-prefix updates to CEF, which it uses to update the FIB and adjacency databases. Existing and
new entries receive the new version ("epoch") number, indicating that they have been refreshed. The
forwarding information is updated on the forwarding engine during convergence. The supervisor engine
signals when the RIB has converged. The software removes all FIB and adjacency entries that have an
epoch older than the current switchover epoch. The FIB now represents the newest routing protocol
forwarding information.
Routing Protocols
Use of the routing protocols require the Enterprise Services Cisco IOS Software image for the
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Catalyst 4500 series switch.
The routing protocols run only on the active supervisor engine, and they receive routing updates from
their neighbor routers. Routing protocols do not run on the standby supervisor engine. Following a
switchover, the routing protocols request that the NSF-aware neighbor devices send state information to
help rebuild the routing tables. Alternately, the IS-IS protocol can be configured to synchronize state
information from the active to the redundant supervisor engine to help rebuild the routing table on the
NSF-capable device in environments where neighbor devices are not NSF-aware. NSF supports the BGP,
OSPF, IS-IS, and EIGRP protocols.
For NSF operation, the routing protocols depend on CEF to continue forwarding packets while the
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routing protocols rebuild the routing information.
BGP Operation
When an NSF-capable router begins a BGP session with a BGP peer, it sends an OPEN message to the
peer. Included in the message is a statement that the NSF-capable device has "graceful" restart
capability. Graceful restart is the mechanism by which BGP routing peers avoid a routing flap following
a switchover. If the BGP peer has received this capability, it is aware that the device sending the message
is NSF-capable. Both the NSF-capable router and its BGP peers need to exchange the graceful restart
capability in their OPEN messages at the time of session establishment. If both peers do not exchange
the graceful restart capability, the session will not be capable of a graceful restart.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
About NSF with SSO Supervisor Engine Redundancy
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