Comprehensive Analysis on product selection of layer-3 Switching Technology

There are many things worth learning about the layer-3 Exchange Technology. Here we mainly introduce the comprehensive analysis of the product selection of the layer-3 Exchange Technology. Simply put, the layer-3 exchange technology is: layer-2 exchange 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) software-based layer-3 exchange technology is relatively simple, but the speed is slow, 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.