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ThinkMo EDU Share – network 44.Brief description of Cisco CEF

蒂娜 No Comments 11/09/2022

ThinkMo EDU Share – network 44.Brief description of Cisco CEF

CEF (Cisco Express Forwarding, Cisco Express Exchange) technology is a brand-new routing switching solution introduced by Cisco. It has good switching performance, enhanced switching architecture and extremely high packet forwarding rate.

The basic functions of traditional routers are route calculation and packet forwarding. They are usually based on a shared memory architecture and use a centralized CPU, that is, a single CPU (or multiple CPUs, connected to form a router cluster) to control the shared bus and connect to multiple interface cards. The card contains simple queues and other structures, communicates with the CPU, and implements packet forwarding through a shared bus. With the rapid development of the Internet and the continuous emergence of a large number of new service requirements, higher requirements are placed on the routing and switching performance of the network. To improve the packet forwarding rate and system performance at the same time, it is necessary to design traditional routers and switching equipment. The architecture is improved, and some new design schemes are added to improve the system performance. Switching routers using CEF technology have qualitative changes in architecture, routing methods and interface card performance. They are especially suitable for the core layer of ISP networks with large traffic, and are also widely used in the backbone of high-speed enterprise networks.

CEF has two modes:

1. Centralized
allows one routing processing module to run express switching, i.e. the FIB and adjacency table reside in the routing processing module and can be used when the line card is unavailable or does not have the ability to decentralize CEF switching Centralized CEF exchange mode.

2. Distributed
(generally denoted as D CEF) allows multiple line cards (VIPs) of the router to run the express switching function separately, provided that the line is a VIP line card or a GSR line card. The central route processor performs functions such as system management/routing and forwarding table calculations, and distributes the CEF tables to individual line cards; each line card maintains an identical copy of the FIB and adjacency table. Line cards perform fast forwarding between port adapters so that switching operations do not require routing switch modules. DCEF employs an “inter-process communication” mechanism to ensure synchronization of the FIB and adjacency lists between the route processor and the interface card.

Exchange algorithm

1.Process switching
The original Cisco router adopts centralized CPU packet switching. All packets are transmitted to the CPU through the shared bus, searched in the routing table, CRC recalculated, and then transmitted to the appropriate line card through the shared bus.

2.The rapid exchange

of IP packets to a specific destination usually causes a packet flow, that is, it is assumed that after a packet to a specific destination is exchanged, another one is likely to arrive soon. By building a cache of exchange targets, the number of times that packets look up the same target in the entire routing table can be reduced. This “route once, then exchange” method is called fast switching. Fast switching greatly improves the packet forwarding rate of the router, and thus becomes a The default switching mechanism on Cisco router platforms. It is important to note, however, that changes to the IP routing table can invalidate the cache, and in environments where routing conditions are constantly changing, the benefits of route caching will be greatly limited.

3.Autonomous switching
Autonomous switching is characterized by offloading some switching functions from the CPU. In effect, the routing cache function is moved from the CPU to the auxiliary switch processor. The received packet on the line card first completes the local routing cache target lookup in the switch processor, and only interrupts the CPU to execute the routing table if the lookup fails. Find. Here, Cisco renames the CPU that periodically calculates the route to the route processor, and the auxiliary switch processor to the switch processor. Implementing autonomous switching on Cisco 7000 series routers can further improve performance such as throughput.

4.Distributed switching
With the introduction of VIP (Versatile Interface Processor, universal interface processor) cards, the switching system of routers gradually develops to a peer-to-peer multi-processor structure. Each VIP card contains a RISC processor, maintains the latest copy of the fast switching cache generated by the routing switch processor, and can independently implement the function of routing switching, completing two types of switching at high speed – local VIP switching and Exchange between VIPs.

5.Express switching
As mentioned above, the caching mechanism of express switching cannot scale well in high-speed dynamic routing environments such as the Internet (there are often network topology changes, routing changes, routing flapping, etc.), and routing changes lead to The cache is invalid, and rebuilding the cache (that is, the process of performing “process exchange”) is computationally expensive; at the same time, with the rapid development of the Internet and its services, various WEB-based applications and interactive services make the number of communications The large number of real-time data streams with short communication time increases, the rapidly exchanged cache contents are constantly changing, and the burden of rebuilding the cache increases, resulting in a decrease in router performance. The CEF express exchange technology is designed and proposed for the above-mentioned shortcomings.

CEF is an advanced Layer 3 switching technology, which is mainly designed for high-performance, highly scalable Layer 3 IP backbone network switching. To optimize the route lookup mechanism of packet forwarding, CEF defines two main components: Forwarding Information base and Adjacency Table.

The Forwarding Information Base (FIB) is a lookup table for routers to determine the target switching. There is a one-to-one correspondence between FIB entries and IP routing table entries, that is, FIB is a mirror of the routing information contained in the IP routing table. Since the FIB contains all the necessary routing information, there is no need to maintain a routing cache. When the network topology or routing changes, the IP routing table is updated, and the content of the FIB changes accordingly.

The CEF utilizes the adjacency table to provide the information needed for the MAC layer rewriting of the packet. Each entry in the FIB points to a next-hop hop in the adjacency list. If the adjacent nodes can realize mutual forwarding through the data link layer, these nodes are listed in the adjacency list.

Once the system finds the adjacency relationship, it writes it into the adjacency table, and the adjacency sequence is generated at any time. Every time an adjacency entry is generated, a link layer header information is pre-computed for that adjacency node, and the link Layer header information is stored in the adjacency table, and when routing is determined, it points to the next network segment and the corresponding adjacency entry. It is then used to encapsulate the packet when CEF is exchanged. To view information about the adjacency table, use the Cisco IOS command: show adjacency/show adjacency detail. When we look at the adjacency table information, we find that there are two main types of adjacencies: Host adjacency and Point to Point. The usual display of the Host adjacency type is an IP address, which indicates the next hop IP address of the adjacency; the display of the Point to Point type is “point 2point”, indicating that this is a point-to-point circuit. In addition, there are other special types, such as Null adjacency, Glen adjacency, etc.

CEF is a high-speed switching method specially designed for high-performance, highly scalable IP backbone networks. It is not difficult to see from the above introduction that CEF can provide unprecedented exchange consistency and stability in large-scale dynamic IP networks. It can effectively make up for the defect of frequent invalidation of fast-switched cache entries. The use of D – CEF distributed switching enables each line card to perform complete switching and provides better performance; CEF takes up less memory than fast-switched routing caches , and can provide load balancing, network accounting and other functions. With CEF Express switching technology and other revolutionary innovations, Cisco’s GSR routers have achieved great success around the world.

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