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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 38 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
Configuring Protocol-Independent Features
This section describes how to configure IP routing protocol-independent features. These features are
available on switches running the IP base or the IP services feature set; except that with the IP base
feature set, protocol-related features are available only for RIP. 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:
• Configuring Distributed Cisco Express Forwarding, page 38-76
• Configuring the Number of Equal-Cost Routing Paths, page 38-77
• Configuring Static Unicast Routes, page 38-78
• Specifying Default Routes and Networks, page 38-79
• Using Route Maps to Redistribute Routing Information, page 38-80
• Configuring Policy-Based Routing, page 38-83
• Filtering Routing Information, page 38-87
• Managing Authentication Keys, page 38-89
Configuring Distributed 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 a Catalyst 3750-E switch stack,
the hardware uses distributed CEF (dCEF) in the stack. 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 and dCEF use the
Forwarding Information Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF and dCEF are the distributed FIB and the distributed adjacency tables.
• 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.
Because the switch or switch stack uses Application Specific Integrated Circuits (ASICs) to achieve
Gigabit-speed line rate IP traffic, CEF or dCEF forwarding applies only to the software-forwarding path,
that is, traffic that is forwarded by the CPU.
CEF or distributed CEF is enabled globally by default. If for some reason it is disabled, you can
re-enable it by using the ip cef or ip cef distributed global configuration command.