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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 22 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
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, refer to the “IP Routing
Protocol-Independent Commands” chapter of the Cisco IOS IP and IP Routing Command Reference for
Release 12.1.
This section includes these procedures:
•
Configuring Cisco Express Forwarding, page 22-53
•
Configuring the Number of Equal-Cost Routing Paths, page 22-54
•
Configuring Static Routes, page 22-55
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Specifying Default Routes, page 22-56
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Redistributing Routing Information, page 22-57
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Filtering Routing Information, page 22-61
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Managing Authentication Keys, page 22-63
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 the Catalyst 3550 switch, the
hardware uses CEF to achieve Gigabit speed line rate IP traffic. 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 uses the Forwarding
Information Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF are the FIB and adjacency tables.
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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.
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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.
•
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, which is recommended, is CEF enabled on all Layer 3 interfaces. You can
disable CEF on an interface by using the no ip route-cache cef interface configuration command; you
can enable CEF on an interface by using the ip route-cache cef interface configuration command.
