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Analysis of Cisco CEF

The basic functions of traditional routers are route calculation and packet forwarding. Usually based on shared memory architecture, a centralized CPU, which is a single CPU (or multiple CPUs, connected into a router cluster) controls the shared bus and connects multiple interface cards. The card contains a simple queue and other structures, communicates with the CPU, and implements packet forwarding through the shared bus.With the rapid development of the Internet and the emergence of a large number of new service requirements, higher requirements are placed on the routing and switching performance of the network. Cisco ccie exam.To improve the packet forwarding rate and system performance at the same time, the design of traditional routers and switching devices must be adopted. The architecture was improved and some new design options were added to improve system performance.Switching routers using CEF technology have qualitative changes in architecture, routing, and interface card performance. They are especially suitable for the core layer of large-volume ISP networks, and are also widely used in the backbone of high-speed enterprise networks.

CEF has two modes:

(1) Centralized:

Allows a routing processing module to run an express exchange,which is the FIB and adjacency tables reside in the routing processing module. When the line card is unavailable or does not have the function of decentralized CEF switching, the centralized CEF switching mode can be used.

(2) Distributed:

(Generally referred to as dCEF) Allows the router’s multiple line cards (VIPs) to run the express switching function separately, provided that the line is a VIP line card or a GSR line card. The central routing processor performs functions such as system management/routing and forwarding table calculations, and distributes the CEF tables to a single line card; each line card maintains the same copy of one FIB and adjacency list.Cisco ccie exam.The line card performs fast forwarding between the port adapters so that the switching operation does not require the participation of the routing switch module. DCEF uses an “internal process communication” mechanism to ensure synchronization of the FIB and adjacency tables between the route processor and the interface card.

Exchange algorithm:

  • Process exchange

The original Cisco routers used centralized CPU packet switching. All packets were passed to the CPU through the shared bus, searched by the routing table, CRC recalculated, and then passed to the appropriate line card through the shared bus.

  • fast exchange

An IP packet arriving at a specific destination address usually causes a packet flow, that is, assuming that the packet has been exchanged to a specific destination, the other is likely to arrive soon.By constructing the cache of the exchange target, it is possible to reduce the number of times the packet finds the same target in the full routing table.Cisco ccie exam. This “one-time route, then exchange” mode is called fast exchange, and the fast exchange greatly improves the packet forwarding rate of the router, thus becoming The default exchange mechanism on the Cisco router platform.However, it should be noted that changes to the IP routing table will invalidate the cache. In an environment where routing conditions are constantly changing, the advantages of the route cache will be greatly limited.

  • Autonomous exchange

The feature of autonomous switching is that some switching functions are offloaded from the CPU. In effect, the routing cache function is moved from the CPU to the secondary switching processor, and the receiving packet on the line card first completes the local routing cache target search in the switching processor, interrupts the CPU if the search fails, and then performs routing.Here, Cisco renames the CPU that periodically calculates the route to the route processor and the auxiliary switch processor to the switch processor. Autonomous switching is performed on the Cisco 7000 Series routers to further improve throughput and other performance.

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