Configuring Protocol-Independent Features; Configuring Cisco Express Forwarding - Cisco 3845 - Security Bundle Router Software Manual
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Chapter 29
Configuring IP Unicast Routing
Configuring Protocol-Independent Features
This section describes how to configure IP routing protocol-independent features. For a complete
description of the IP routing protocol-independent commands in this chapter, see the "IP Routing
Protocol-Independent Commands" chapter of the Cisco IOS IP Command Reference, Volume 2 of 3:
Routing Protocols, Release 12.2.
These sections contain this configuration information:
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Configuring Cisco Express Forwarding
Cisco Express Forwarding (CEF) is a Layer 3 IP switching technology used to optimize network
performance. CEF implements an advanced IP look-up and forwarding algorithm to deliver maximum
Layer 3 switching performance. CEF is less CPU-intensive than fast switching route caching, allowing
more CPU processing power to be dedicated to packet forwarding. In dynamic networks, fast switching
cache entries are frequently invalidated because of routing changes, which can cause traffic to be process
switched using the routing table, instead of fast switched using the route cache. CEF use the Forwarding
Information Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF are the distributed FIB and the distributed adjacency tables.
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Because the switch uses Application Specific Integrated Circuits (ASICs) to achieve Gigabit-speed line
rate IP traffic, CEF forwarding applies only to the software-forwarding path, that is, traffic that is
forwarded by the CPU.
CEF is enabled globally by default. If for some reason it is disabled, you can re-enable it by using the ip
cef global configuration command.
The default configuration is CEF enabled on all Layer 3 interfaces. Entering the no ip route-cache cef
interface configuration command disables CEF for traffic that is being forwarded by software. This
command does not affect the hardware forwarding path. Disabling CEF and using the debug ip packet
detail privileged EXEC command can be useful to debug software-forwarded traffic. To enable CEF on
an interface for the software-forwarding path, use the ip route-cache cef interface configuration
command.
OL-23400-01
Configuring Cisco Express Forwarding, page 29-93
Configuring the Number of Equal-Cost Routing Paths, page 29-94
Configuring Static Unicast Routes, page 29-95
Specifying Default Routes and Networks, page 29-96
Using Route Maps to Redistribute Routing Information, page 29-97
Filtering Routing Information, page 29-100
Managing Authentication Keys, page 29-103
The FIB is similar to a routing table or information base and maintains a mirror image of the
forwarding information in the IP routing table. When routing or topology changes occur in the
network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB
maintains next-hop address information based on the information in the IP routing table. Because
the FIB contains all known routes that exist in the routing table, CEF eliminates route cache
maintenance, is more efficient for switching traffic, and is not affected by traffic patterns.
Nodes in the network are said to be adjacent if they can reach each other with a single hop across a
link layer. CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency
table maintains Layer 2 next-hop addresses for all FIB entries.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Configuring Protocol-Independent Features
29-93
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