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NetFlow switching can also be configured on VIP interfaces. For each of these forwarding processes, designers should consider the impact of access lists. At present, NetFlow typically provides the best performance when access lists are needed. A recent study mentioned in an article by Peter Morrissey in Network Computing demonstrated a 700 percent performance benefit when using NetFlow and a 200-line access list. Performance benefits are lower with shorter lists; however, with anything beyond a single-line access list, NetFlow will yield better performance than optimal switching.

Cisco Express Forwarding

Cisco Express Forwarding (CEF) is a switching function, designed for high-end backbone routers. It functions on Layer 3 of the OSI model, and its biggest asset is the capability to remain stable in a large network. However, it’s also more efficient than both the fast- and optimum-switching defaults. CEF is wonderfully stable in large environments because it doesn’t rely on cached information. Instead of using a CEF cache, it refers to the Forwarding Information Base (FIB), which consists of information duplicated from the IP route table. Every time the routing information changes, the changes are propagated to the FIB. Thus, instead of comparing old cache information, a packet looks to the FIB for its forwarding information.

CEF stores the Layer 2 MAC addresses of connected routers (or next-hop) in the adjacency table. Even though CEF features advanced capabilities, you should consider several restrictions before implementing CEF on a router. According to the document “Cisco Express Forwarding,” available from the Cisco Web page Cisco Connection Online, system requirements are quite high. The processor should have at least 128MB of RAM, and the line cards should have 32MB each. CEF takes the place of VIP distributed- and fast-switching on VIP interfaces. The following features aren’t supported by CEF:

  Token Ring
  Multipoint PPP
  Access lists on the GSR (Giga Switch Router)
  Policy routing

Nevertheless, CEF does many things—even load balancing is possible through the FIB. If there are multiple paths to the same destination, the IP route table knows about them all. This information is also copied to the FIB, which CEF consults for its switching decisions.

Load balancing can be configured in two different modes. The first mode is load balancing based on the destination (called per-destination load balancing); the second mode is based on the packet (called per-packet load balancing). Perdestination load balancing is on by default and must be turned off to enable per-packet load balancing.

Accounting may also be configured for CEF, which furnishes you with detailed statistics about CEF traffic. You can make two specifications when collecting CEF statistics:

  To collect information on traffic that’s forwarded to a specific destination
  To collect statistics for traffic that’s forwarded through a specific destination

CEF was designed for large networks—if reliable and redundant switching paths are necessary, CEF is certainly preferred. However, there are significant hardware requirements, and some Cisco IOS features may not be available.

Cisco routers may support concurrent load balancing when routing IP. However, this feature is dependent on the switching mechanism in use. Up to six paths may be balanced in the current releases of the IOS, dependent on the routing protocol in use.

Autonomous and silicon switching have been updated with optimum, distributed, and NetFlow. However, from a load-balancing perspective, they operate in the same manner as their replacements. Autonomous and silicon-switched packets will be load-balanced by destination.


This chapter presented a wide array of material on the IP protocol and on some of the criteria for selecting an IP routing protocol. The next chapter will build upon this material and provide greater depth regarding the options available to designers regarding IP routing protocols.

Readers should feel comfortable with the following concepts:

  IP address structures
  IP address classes
  IP address summarization
  The implications of RFC 1918/RFC 1597
  The methods used by the router to forward packets
  The role of the router and its additional features
  The problems associated with discontiguous subnets and the benefits of VLSM-aware protocols

Designers should also be prepared to integrate this material into the following chapter, which details the IP routing protocols, and subsequent ones, which address non-IP-based protocols and the issues that confront designers in typical networks.

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