CEF Overview

Cisco Express Forwarding is enabled by default on most Cisco platforms running Cisco IOS software Release12.0 or later. When Cisco Express Forwarding is enabled on a router, the Route Processor (RP) performs the express forwarding.

To find out if Cisco Express Forwarding is enabled on your platform, enter the show ip cefcommand. If Cisco Express Forwarding is enabled, you receive output that looks like this:

Router# show ip cef 
Prefix              Next Hop            Interface
[...]
10.2.61.8/24        192.168.100.1       FastEthernet1/0/0
                    192.168.101.1       FastEthernet6/1
[...]

If Cisco Express Forwarding is not enabled on your platform, the output for the show ip cefcommand looks like this:

Router# show ip cef
%CEF not running

Distributed Cisco Express Forwarding is enabled by default on the Catalyst 6500 series switch, the Cisco 7500 series router, and the Cisco 12000 Series Internet Router. When distributed Cisco Express Forwarding is enabled on your platform, the line cards perform the express forwarding.

If Cisco Express Forwarding is not enabled on your platform, use the ip cefcommand to enable (central) Cisco Express Forwarding or the ip cef distributed command to enable distributed Cisco Express Forwarding.

• Improved performance--Cisco Express Forwarding is less CPU-intensive than fast switching route caching. As a result, more CPU processing power can be dedicated to Layer 3 services such as quality of service (QoS) and encryption.
• Scalability--Cisco Express Forwarding offers full switching capacity at each line card when distributed Cisco Express Forwarding mode is active. Distributed Cisco Express Forwarding is a distributed switching mechanism that scales linearly with the number of interface cards and the bandwidth installed in the router.
• Resilience--Cisco Express Forwarding offers an unprecedented level of switching consistency and stability in large dynamic networks. In dynamic networks, fast-switched cache entries are frequently invalidated by routing changes. These changes can cause traffic to be process-switched through use of the routing table, rather than fast switched through use of the route cache. Because the forwarding information base (FIB) lookup table contains all known routes that exist in the routing table, it eliminates the need for route cache maintenance and the steps involved with fast-switch or process-switch forwarding. Cisco Express Forwarding can switch traffic more efficiently than typical demand caching schemes.

You can use Cisco Express Forwarding in any part of a network. For example, the figure below shows Cisco Express Forwarding being run on routers at aggregation points at the core of a network where traffic levels are high and performance is critical.

Figure 1

Cisco Express Forwarding Example

Cisco Express Forwarding in platforms at the network core provides the performance and scalability that networks need to respond to continued growth and steadily increasing network traffic. Cisco Express Forwarding is a distributed switching mechanism that scales linearly with the number of interface cards and the bandwidth installed in the router.

Cisco Express Forwarding currently supports the following media:

• ATM/AAL5snap, ATM/AAL5mux, and ATM/AAL5nlpid
• Ethernet
• FDDI
• Frame Relay
• High-Level Data Link Control (HDLC)
• PPP
• Spatial Reuse Protocol (SRP)
• TokenRing
• Tunnels

Information conventionally stored in a route cache is stored in several data structures for Cisco Express Forwarding switching. The data structures provide optimized lookup for efficient packet forwarding. The two main components of Cisco Express Forwarding operation are the forwarding information base (FIB) and the adjacency tables.

The FIB is conceptually similar to a routing table or information base. A router uses this lookup table to make destination-based switching decisions during Cisco Express Forwarding operation. The FIB is updated when changes occur in the network and contains all routes known at the time. For more information, see the FIB Overview section.

Adjacency tables maintain Layer 2 next-hop addresses for all FIB entries. For more information, see the CEF Adjacency Tables Overview.

This separation of the reachability information (in the Cisco Express Forwarding table) and the forwarding information (in the adjacency table), provides a number of benefits:

• The adjacency table can be built separately from the Cisco Express Forwarding table, allowing both to be built without any packets being process switched.
• The MAC header rewrite used to forward a packet is not stored in cache entries, so changes in a MAC header rewrite string do not require invalidation of cache entries.

Cisco Express Forwarding uses a FIB to make IP destination prefix-based switching decisions.

The FIB contains the prefixes from the IP routing table structured in a way that is optimized for forwarding. 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 there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for the route cache maintenance that is associated with switching paths such as those used in fast switching and optimum switching.

• CEF FIB and Load Balancing
  

A node is said to be adjacent to another node if the node can be reached with a single hop across a link layer (Layer 2). Cisco Express Forwarding stores forwarding information (outbound interface and MAC header rewrite) for adjacent nodes in a data structure called the adjacency table. Cisco Express Forwarding uses adjacency tables to prepend Layer 2 addressing information to packets. The adjacency tables maintain Layer 2 next-hop addresses for all FIB entries.

The following sections provide additional information about adjacencies:

• Adjacency Discovery
  
• Adjacency Types That Require Special Handling
  
• Unresolved Adjacency
  

You can use central Cisco Express Forwarding mode when line cards are not available for Cisco Express Forwarding switching, when you need to use features not compatible with distributed Cisco Express Forwarding switching, or when you are running on a nondistributed platform. When central Cisco Express Forwarding mode is enabled, the Cisco Express Forwarding FIB and adjacency tables reside on the RP, and the RP performs the express forwarding.

The figure below shows the relationship between the routing table, the FIB, and the adjacency table during central Cisco Express Forwarding mode operation. The Catalyst switches forward traffic from workgroup LANs to a Cisco 7500 series router on the enterprise backbone running central Cisco Express Forwarding. The RP performs the express forwarding.

Figure 2

Central Cisco Express Forwarding Mode Operation

For additional scalability, Cisco Express Forwarding runs in the distributed Cisco Express Forwarding form on certain platforms by spreading processing tasks across two or more line cards. When distributed Cisco Express Forwarding mode is enabled, line cards maintain identical copies of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters, relieving the RP of involvement in the switching operation, thus also enhancing system performance.

Distributed Cisco Express Forwarding uses an interprocess communication (IPC) mechanism to ensure synchronization of FIB tables and adjacency tables on the RP and line cards.

The figure below shows the relationship between the RP and line cards when distributed Cisco Express Forwarding mode is active.

Figure 3

Distributed Cisco Express Forwarding Mode Operation

In the Cisco 12000 Series Internet Router, shown in the figure above, the line cards perform the switching. In other routers where you can mix various types of cards in the same router, all cards might not support distributed Cisco Express Forwarding. When a line card that does not support distributed Cisco Express Forwarding receives a packet on one of these other routers, the line card forwards the packet to the next higher switching layer (the RP). This structure allows legacy interface processors to exist in the router with newer interface processors.

Note	The Cisco 12000 Series Internet routers operate only in distributed Cisco Express Forwarding mode.

• Per-destination load balancing and the universal load sharing algorithm
• Distributed tunnel switching
• Multipoint generic routing encapsulation (GRE) tunnels

• CEF Distributed Tunnel Switching
  
• CEF-Switched Multipoint GRE Tunnels
  

The table below contains links to information about features that you can configure for use with Cisco Express Forwarding or distributed Cisco Express Forwarding operation.