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| 5.2 |  | Cisco Express Forwarding |
| 5.2.7 |  | Additional benefits of CEF-based forwarding |

In addition to the benefits described previously, CEF-based forwarding provides several other benefits. These benefits include enhanced scalability, network stability, load balancing, ACL processing, and multicasting. Many of these benefits are supported only on the Catalyst 6500 series (a subset of the Catalyst 6000 family). The following sections briefly describe each of these additional benefits.

Scalability
CEF increases scalability not only by the raw number of available FIB entries, but also by the replication of CEF-based forwarding technology on a per-slot basis. This “distributed” capability (dCEF) allows the Catalyst 6500 series to provide sufficient forwarding capabilities for the largest of networks. The Catalyst 6500 series, when equipped with a Supervisor Engine 2, can provide as many as 256,000 entries in the FIB table and 256,000 entries in the adjacency table. These entries are not stored via a traditional hashing algorithm, as is the case with MLS.

Availability
Although the Catalyst 6000 family offers a breadth of features to support high availability, the addition of CEF further increases the availability and stability demanded in today’s enterprise networks. As networks increase in size, a natural side effect is the increased chance of instability and change. Network instability, whether due to failures, configuration changes, or bursty traffic patterns, can have a tremendous impact on routing implementations, which rely on heavy CPU computations for routing-table maintenance. Because CEF employs a mechanism by which the forwarding information is constructed based on the network topology rather than a representation of traffic within the network, the CPU is no longer burdened by having to set up large umbers of entries. This also means that network availability is not directly linked to the actual size of the network.

Access Control Lists
Another manner in which various CEF implementations vary is with respect to their support of ACLs. In the case of the Catalyst 6500 series, ACLs can be handled in hardware for most common configurations. CEF-based forwarding does not have any impact on this functionality. The most notable ACL options that disable hardware processing for a given ACL are the use of the log keyword or enabling “ip unreachables” on a given interface (which enable the sending of ICMP unreachable messages).

Multicasting
The CEF implementation on the Catalyst 6000 family also includes support for IP multicast. The FIB table can hold as many as 16,000 IP multicast entries.

In conclusion, CEF provides high performance and high scalability for the largest and most demanding networks. The Supervisor Engine 2 offers the scalability and performance requirements necessary for both large enterprise and service-provider environments. Cisco has also managed to maintain the rich accounting support of the MLS model (utilizing NetFlow features) while introducing CEF-based forwarding.