Experts tell you what a layer-3 switch is.

Source: Internet
Author: User

Experts tell you what a layer-3 switch is. Now every household has their own computer and understands what a layer-3 switch is. At that time, the layer-3 switch was based on software and the forwarding speed was very slow, later, it was developed to implement layer-3 switching with hardware.

In, a layer-3 switch was born. So what is a layer-3 switch? In the early days, people wanted to combine the layer-2 switching and layer-3 routing functions on a single device to reduce the number of devices. At that time, layer-3 switching was based on software, and the forwarding speed was very slow. Later, it was developed to implement layer-3 switching with hardware.

From today's point of view, a layer-3 switch is essentially a special type of router, which focuses on switching layer-2 and layer-3 in terms of performance. It has a strong exchange capability and a low-cost router. It implements layer-3 switching of IP packets with ASIC, and its exchange capability is above MPPS. Traditional routers generally do not have more than 0.1 million packets/second. This refers to the forwarding capability of a single board, it does not mean the overall forwarding capability of multiple boards in Distributed forwarding situations, nor does it include GSR consisting of expensive network processors. In addition, a higher-speed CPU is now available, but it is hard to exceed 1 mpps3 ).

The high price of the network processor lies in its complexity in addition to layer-3 switching. It also has strong QOS, POLICY, and other functions. Taking IBM's Rainer processor as an example: its hardware can manage thousands of streams, the software configures bandwidth of different streams, and is embedded with PowerPC processors. It has a large number of coprocessors and hardware accelerators, data can be processed in parallel.

In order to reduce costs, the forwarding part of a layer-3 Switch cannot support thousands of streams at a wire speed and can allocate bandwidth. In the past, some people thought that it was very expensive to use the longest match to search for a large route table. In other words, it was very expensive to use hardware to achieve the longest match for a large route table. According to the survey data, this is not necessarily true.

The cost of using CAM for route table search is not high. Take KawasakiLSI's CAM chip KE5BLME064 that supports longest matching search as an example. It supports 64 K route prefix items. Each ENTRY40bit is wide and the packet search speed can reach 6.7 Mpps, with a latency of several hundred nanoseconds, the price is less than 60 USD 6.

Of course, reducing the supported route table items will undoubtedly reduce the cost, and there is no need for too many route table items in the general application environment of layer-3 switches. Therefore, three-layer switches generally support less route table items than GSR, for example, the CISCO4000 series only supports 16-to 32 K route table items, and the ACCLER1000 of Nortel supports 32 K.

However, the SUPERENGINES2 of CISCO applications in the 6500 series already supports up to 128 kb, which can be equivalent to the table items of the backbone network router. The route tables supported by the forwarding engine designed by CISCO are becoming larger and larger, considering that layer-3 switches are applied to man networks and backbone networks.

In general, the price of a network processor should be based on its flexibility. You must support QOS, POLICY, and multiple protocols. In particular, some protocols have not yet fully formed standards, the network processor may only need to modify the software to support most layer-3 switches, which also leads to incompatibility between the early three-layer Switches of many manufacturers and the current protocol ), the Network Processor needs to identify more packets and schedule more complex tasks to achieve line rate processing.

It is often composed of multiple micro engines and supports multi-thread processing. These may be the main reason for the sharp rise in network processor prices. In contrast, layer-3 switches have weak capabilities in this regard. For example, the CISCO6000 series only supports 16 QOS queues, 4000 only supports one queue, and 5500 supports two queues, the layer-3 engine of Extreme only supports four queues.

Because hardware is used to implement layer-3 switching, the switching speed can be very high, but to support a large number of layer-3 protocols, such as IP, IPX, AppleTalk, DECnet, and so on, these protocols have different packaging formats, it is easy to implement with software, but it is very difficult to implement with hardware. There are too many protocols to implement forwarding with hardware, which will only bring about a sharp increase in costs.

What is a layer-3 Switch? Generally, only common IP addresses, IPX protocols, and IP multicast are supported. Some layer-3 switches support others, but they are implemented by software with a low forwarding rate. For example, CISCO's ipvst6000 series uses software to implement AppleTalk on MSFC, the forwarding rate ranges from 0.1 million to 0.2 million packets per second.

What is a layer-3 switch now mainly provides Ethernet interfaces. Ethernet technology is simple and cost-effective. It is the most widely used in enterprise networks. layer-3 switches use Ethernet interfaces to provide low-cost and high-speed switches for enterprise networks. They replace vrouters with high prices but poor performance.

The ASIC of a layer-3 Switch provides both layer-2 and layer-3 IP exchanges, integrating the functions of the original layer-2 Ethernet switch and vro, greatly reducing users' costs. With the rise of broadband services, the application scope of layer-3 switches is constantly expanding, especially the rapid development of GE and 10G Ethernet technologies. As a result, layer-3 switches have gradually moved from enterprise networks to man networks, the expansion of application scope also promotes its design innovation.

For example, CACHE technology is widely used in layer-3 switches. When layer-3 switches are applied to man networks and backbone networks, the business flow becomes more irregular and network topology changes increase, due to the increase in video language services and the demand for QOS, the disadvantages of adopting this technology are becoming more and more obvious;

What is layer-3 switches that require more and more protocols to be supported when developing towards man and backbone networks? For example, some manufacturers already support OSPF, BGP, MPLS, and other protocols, some even use programmable ASIC to support IPV6.

Layer-3 switches support more and more interfaces, and some already support Wide Area Network Ports. For example, the CISCO6000 series provides the Wide Area Network Module flexWAN and various wide area network interfaces. Such as ATM, POS, callback relay, and PPP. However, this part is also handled by software like traditional routers. flexWAN has two 263M RM7000 processors to process these wan interface protocols, and the interface speed is not high, currently only OC-3ATM or OC-3POS is supported.

The adapters of these Wan interfaces are compatible with the CISCO7200/7500 series, but you can be sure that if you want to use the WAN interface on a layer-3 switch, the price of the WAN interface on a common router should be similar, because the same technology is used on interfaces. However, as a result, a layer-3 Switch leads to a lack of outlets for Routers such as CISCO7000, which have always provided Wan interfaces. Some people have commented that these routers are ending.

Generally, a layer-3 switch does not support wide-area network ports. Another reason is that MTU may be different for different wide-area network ports. In this way, to achieve line rate forwarding, you must support partitions on the hardware, this will lead to complicated design. Currently, layer-3 switches do not support sharding, and Ethernet interfaces are used on all switches.

However, these manufacturers only provide a variety of Wan interfaces and Protocol modules to enhance the selling points of layer-3 switches. They cannot provide layer-2 high-speed forwarding from hardware, however, a layer-3 Switch cannot fully utilize the layer-3 switch function.

The benefit of adding a WAN interface is that if a layer-3 Switch needs to be connected to the WAN over the internet, it will be more flexible and not necessarily add a level-1 router at the exit, at present, many manufacturers do not support layer-3 switches for wide-area network ports. As a result, a router is required to connect to the internet during networking. Of course, this is not entirely because of interfaces, there are also limited protocols supported by these layer-3 switches, such as BGP protocol not supported, insufficient route table items supported, and imperfect firewall functions.

A layer-3 switch is divided into multiple grades based on application scenarios. The performance and price are different and cannot be generalized. Layer-3 switches applied on man networks or backbone networks require more functions on layer-3 switches. Increasing the design complexity will inevitably increase the price, A particularly high-end Layer-3 switch is no different from GSR.

It should be said that if a layer-3 switch reaches this level, it cannot be called a layer-3 Switch. Like a high-end router of JUNIPER, its main components are all ASIC-designed, we call it a layer-3 switch, but its ASIC features are too complex, too powerful, and very expensive. It is more appropriate to call it a high-end router.

Generally, a layer-3 switch is called a switch because it has limited functions and is mainly for exchange-oriented purposes. Its design is mainly designed for enterprise networks and man networks, and the number of requirements on this layer is met. For users at this level, while meeting the main performance, cost is the most important factor to consider. It uses sufficient power as the criterion and does not emphasize the implementation of excessive protocols on layer-3 switches.

Provides many functions that cannot be used at all. For example, implementing powerful QOS and supporting BGP on a layer-3 switch is practically useless to most users. At present, when a man is built in China, it does not have to consider too much QOS. At present, it is mainly on NTERNET. This is actually in line with China's national conditions. For VOD, renting a disc in China requires only one or two yuan or even a few cents. vcdns are widely used in cities, how many people are willing to go to VOD?

To understand the application scope of a layer-3 Switch and to meet the requirements of low cost, we can understand why the layer-3 Switch has weak functions in Cos and POLICY, mainly providing Ethernet ports and limited support services, limited protocols and limited routing functions. A layer-3 Switch provides Ethernet interfaces, and Ethernet interfaces of different rates have similar frame formats.

It is easy to use hardware to implement layer-2 high-speed forwarding. Generally, the layer-2 forwarding function far exceeds the layer-3 function. Many layer-3 switches have a layer-2 forwarding capability of 100 MPPS, while the layer-3 forwarding capability is only a few MPPS. GE and 10GE provide good conditions for applications of L3 switches on the Wide Area Network at a low price. There is no difference in what layer-3 switches use the same technology and vro on the exchange network. For example, the CISCO6500 series uses CROSSBAR technology.

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