IntroductionThis document describes Cisco Express Forwarding (CEF) Show PrerequisitesRequirementsThere are no specific requirements for this document. Components UsedThis document is not restricted to specific software and hardware versions. The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command. ConventionsFor more information on document conventions, see the Cisco Technical Tips Conventions . OverviewCisco Express Forwarding (CEF) switching is a proprietary form of scalable switching intended to tackle the problems associated with demand caching. With CEF switching, the information which is conventionally stored in a route cache is split up over several data structures. The CEF code is able to maintain these data structures in the Gigabit Route Processor (GRP), and also in secondary processors such as the line cards in the 12000 routers. The data structures that provide optimized look-up for efficient packet forwarding include:
CEF can be enabled in one of two modes:
For more information about CEF switching, see Cisco Express Forwarding (CEF) White Paper . CEF OperationsUpdate the GRPs Routing TablesFigure 1 illustrates the process by which a routing update packet is sent to the Gigabit Route Processor (GRP) and the resultant forwarding update messages are sent to FIB tables on the line cards. For clarity, the numbering of the next paragraphs corresponds to the numbering in Figure 1. The next process occurs during route table initialization, or any time the network topology changes (when routes are added, removed, or changed). The process shown in Figure 1 involves five main steps:
Figure 1: Path Determination and Layer 3 Switching Diagram Path Determination and Layer 3 Switching Diagram Packet Forwarding for all Line Cards Except OC48 and QOC12Once the line cards have enough forwarding information to determine the path through the switching fabric (for instance, the destination of the next hop), the 12000 router is ready to forward packets. The next steps outline the simple and fast forwarding technique used by the 12000 router (see Figure 1). For clarity, the lettering of the paragraphs corresponds to the lettering in Figure 1.
The advantage of this simple forwarding process is that most data transmission tasks can be done in ASICs, and allows the 12000 to operate at gigabit rates. Also, data packets are never sent to the GRP. Packet Forwarding for OC48 and QOC12 Line CardsWhen the line cards have enough forwarding information to determine the path through the switching fabric (for example, the destination of the next hop), the 12000 router is ready to forward packets. The next steps make up the simple and hyper-fast forwarding technique used by the 12000 (see Figure 2). For clarity, the lettering of the paragraphs corresponds to the lettering in Figure 2.
The advantage of the new forwarding process is that it optimizes the card specifically for faster speeds, such as the OC48/STM16. Figure 2: Packet Switching for Faster Line Cards Packet Switching for Faster Line Cards Related Information
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