Viewpoint: Analysis of layer-3 Switch Technical Principles

There are many things worth learning about layer-3 switches. Here we mainly introduce the technical principles of layer-3 switches. In short, layer-3 switching technology is layer-2 switching technology + layer-3 forwarding technology. It solves the network bottleneck caused by low speed and complexity of traditional routers.

What is layer-3 switching?

Layer-3 switching is also called multi-layer switching technology or IP address switching technology. As we all know, the traditional exchange technology operates on the data link layer, the second layer of the OSI network standard model, while the third layer exchange technology implements high-speed data packet forwarding in the network model. To put it simply, layer-3 switching technology is layer-2 switching technology + layer-3 forwarding technology. The emergence of layer-3 switching technology solves the problem that the subnet must be managed by routers after the network segments are divided in the LAN, and solves the network bottleneck caused by the low speed and complexity of traditional routers.

Layer-3 Switching Principle

A device with layer-3 switching is a layer-2 switch with layer-3 routing, but it is an organic combination of the two, it is not simply to overlay the hardware and software of a router device on a LAN switch.

The principle is: Assume that two sites A and B that use the IP protocol communicate with each other through the layer-3 Switch. When sending Site A, compare its IP address with the IP address of site B, determine whether Site B is in the same subnet as itself. If the destination site B and the destination site A are in the same subnet, Layer 2 Forwarding is performed. If two sites are not in the same subnet, if Station A needs to communicate with Station B, station A must send an ARP (Address Resolution) packet to the "Default Gateway, the IP address of the "Default Gateway" is actually a layer-3 Switch module. When station A broadcasted an ARP request to the IP address of the "Default Gateway", if the layer-3 Switching Module knew the MAC address of Station B during the previous communication, then the MAC address of B is returned to the sending Site. Otherwise, the layer-3 Switch module broadcasts an ARP request to Site B based on the route information. Site B receives the ARP request and then replies to the layer-3 Switch module with its MAC address, the layer-3 Switching Module saves the address and sends it back to the sender Site A. It also sends the MAC address of Site B to the MAC address table of the layer-2 switching engine. After that, when A sends all the packets to B, the information can be exchanged at A high speed. Because layer-3 processing is only required in the routing process, most of the data is forwarded through layer-2 switch, so the speed of layer-3 switch is very fast, close to the speed of layer-2 switch, and the price is much lower than that of the same router.

L3 Switch Type

Layer-3 switches can be divided into pure hardware and pure software based on their data processing.

1) Hardware-only layer-3 technology is relatively complex, costly, but fast, with good performance and strong load capabilities. The principle is to use an ASIC chip and hardware to search and refresh route tables. When the data is received by the port interface chip, first find the target MAC address in the layer-2 switching chip. If yes, perform layer-2 forwarding; otherwise, send the data to the layer-3 engine. In the layer-3 engine, the ASIC chip searches for the corresponding route table information, compares to the destination IP address of the data, and then sends the ARP packet to the destination host to obtain the MAC address of the host, send the MAC address to a layer-2 chip and the layer-2 Chip forwards the packet.

2) The software-based layer-3 switch technology is relatively simple, but it is slow, and is not suitable for cooperation. The principle is to use the CPU software to find the route table.

When the data is received by the port interface chip, first find the target MAC address in the layer-2 switching chip. If the data is found, perform layer-2 forwarding. Otherwise, the data is sent to the CPU. The CPU looks for the corresponding route table information, which is opposite to the destination IP address of the Data. Then, it sends the ARP packet to the destination host to obtain the MAC address of the host and sends the MAC address to the L2 chip, the second-Layer Chip forwards the packet. Because low-cost CPU processing is slow, the processing speed of such a layer-3 switch is slow.

Market product selection

In recent years, the construction of Broadband IP networks has become a hot topic. Next we will introduce some layer-3 switches that are suitable for the access layer or small and medium size convergence layer. Mainstream third-layer switches on the market include Cisco's ipvst2948gl3, Extreme's Summit24, and AlliedTelesyn's Rapier24. These three-layer switches have their own unique features, covers most of the application features of layer-3 switches. Of course, when selecting a layer-3 switch, you can determine and select the products of the above products or other manufacturers based on your own needs, for example, Passport/Acceler series of Nortel Networks, the original Cabletron SSR series after Cabletron Release 4, most of the SSR L3 switches have been incorporated into Riverstone), Avaya's CajunM series, and 3Com's Superstack34005 series. In addition, Chinese network manufacturers such as shenzhoudigital network, TCL network, Shanghai Radio and Television yingzhi, Ziguang network, and shouxin have launched three-layer switch products. The following describes three of these products, so that you can fully understand the layer-3 Switch and select an appropriate model based on your situation.

In short, three-tier switches have been widely used since the concept was proposed. Although they have only been used for a few years, their extended functions have been enriched with practical applications. With the development of ASIC hardware chip technology and the promotion of practical application, layer-3 switching technology and products will also be further developed.