1. Introduction Traditional routers function as network isolation devices, broadcast isolation devices, route forwarders, and firewalls. However, with the continuous development of networks, the load on routers is rapidly increasing. One important reason is the widespread use of VLAN (Virtual Local Area Network) technology for security and management convenience. VLAN technology can logically isolate different network segments, ports, and even hosts, and communication between different VLANs must be forwarded through routers. Because of the large data traffic in LANs, a large amount of information exchange between VLANs must be forwarded through routers. As data traffic continues to grow, routers become network bottlenecks. To solve this bottleneck in LANs, many enterprises, schools, and residential communities use Layer 3 switches when building LANs. Layer 3 switching technology introduces switching technology to the network layer, and the application of Layer 3 switches has penetrated from the backbone and aggregation layers in the network center all the way to the access layer at the network edge. 2. Layer 3 Switching Technology 2.1 Concept of Layer 3 Switching Layer 3 switching technology, also known as IP switching technology or high-speed routing technology, is a concept proposed in contrast to traditional switching. As is well known, traditional switching technology operates at Layer 2—the data link layer—in the OSI network standard model, while Layer 3 switching technology achieves high-speed packet forwarding at Layer 3 of the network model. Simply put, Layer 3 switching technology is: Layer 2 switching technology + Layer 3 forwarding technology. This is a mechanism that utilizes information from Layer 3 protocols to enhance Layer 2 switching functionality. A device with Layer 3 switching capabilities is a Layer 2 switch with Layer 3 routing capabilities, but it is an organic combination of the two; it is not simply a matter of superimposing router hardware and software onto a LAN switch. 2.2 The Principle of Layer 3 Switching From a hardware implementation perspective, currently, Layer 2 switch interface modules exchange data via a high-speed backplane/bus. In Layer 3 switches, the Layer 3 routing hardware modules related to routers are also plugged into the high-speed backplane/bus. This allows the routing module to exchange data at high speed with other modules that require routing, thus breaking through the traditional limitations of external router interface speeds (10 Mbit/s to 100 Mbit/s). In terms of software, Layer 3 switches redefine traditional software-based routers: (1) Forwarding of data packets: such as forwarding of IP/IPX packets, these regular processes are implemented at high speed through hardware; (2) Layer 3 routing software: such as updating routing information, maintaining routing tables, calculating routes, determining routes, etc., are implemented using optimized and efficient software. Assume there are two sites using the IP protocol, and the communication process through a Layer 3 switch is as follows: If the sending site A knows the IP address of the destination site B when it starts sending, but does not yet know the MAC address it needs to send on the local area network, then it needs to use Address Resolution Protocol (ARP) to determine the MAC address of B. A compares its own IP address with the IP address of B, and uses the subnet mask configured in its software to extract the network address to determine whether B is in the same subnet as itself. If B and A are on the same subnet, A broadcasts an ARP request, B returns its MAC address, A caches this address, and uses it to forward data packets. The Layer 2 switching module looks up the MAC address table to determine the destination port for the data packet. If the two stations are not on the same subnet, A sends an ARP (Address Resolution Protocol) packet to the "default gateway," whose IP address is already set in the system software. This IP address actually corresponds to the Layer 3 switching module of the Layer 3 switch. When A broadcasts an ARP request to the IP address of the "default gateway," if the Layer 3 switching module has already obtained B's MAC address in previous communications, it replies to the sending station A with B's MAC address; otherwise, the Layer 3 switching module broadcasts an ARP request to the destination station based on routing information. B receives this ARP request and replies to the Layer 3 switching module with its MAC address. The Layer 3 switching module saves this address and replies to the sending station A. In the future, when data packets are forwarded between A and B, the MAC address of the final destination site will be used to encapsulate the packets, and the data forwarding process will be handled by the second-layer switch, so that information can be exchanged at high speed [1]. 2.3 Features of the third-layer switch The prominent features are as follows: (1) Organic hardware combination makes data exchange faster; (2) Optimized routing software makes routing process more efficient; (3) Except for the necessary routing decision process, most of the data forwarding process is handled by the second-layer switch; (4) When multiple subnets are interconnected, it is only a logical connection with the third-layer switch module, unlike the traditional external router which requires additional ports, thus protecting the user's investment. The goal of the third-layer switch is that as long as there is a more direct second-layer path between the source address and the destination address, there is no need to forward data packets through the router. The third-layer switch uses the third-layer routing protocol to determine the transmission path. This path can be used only once or stored for later use. Then the data packets are sent quickly by bypassing the router through a virtual circuit. The emergence of Layer 3 switching technology has solved the problem of subnets within a local area network (LAN) relying on routers for management after network segmentation, thus resolving network bottlenecks caused by the low speed and complexity of traditional routers. Of course, Layer 3 switching technology is not simply a combination of network switches and routers, but rather an organic integration of the two, forming an integrated and complete solution. 3. Design of a Layer 3 Switch Considering market needs, switch costs, and proprietary intellectual property rights, we designed and developed the VLSW 4150 series switches with independent intellectual property rights. The VLSW 4150 series switches provide enterprises with a high-performance, multi-layered solution. The VLSW 4150 is also suitable for ISPs and service providers, especially large operators, enhancing their competitiveness in the IP market. 3.1 Overall Design The VLSW 4150 switch has 24 RJ45 10/100BASE-TX auto-sensing ports, providing two optional fiber optic 10/100BaseTx Ethernet ports, 100BaseFx Fast Ethernet ports, or Gigabit Ethernet ports (SX, LX, ZX), and additionally one serial port and one 100M Ethernet port for switch configuration. The VLSW 4150 switch architecture supports speeds up to 11Gbps, with multilayer switching speeds reaching 6.6Mpps; it supports 8,192 MAC addresses; for better network traffic control and network security, it also supports rate limiting in 1M increments; it supports Tagged VLANs and MAC-based frame filtering, as well as RIP, OSPF, and BGP routing protocols. The VLSW 4150 switch provides stacking technology, allowing multiple switches to be managed with a single logical IP address, and can mirror packets from other switches on a single port. It provides a web-based management system and CLI for switch debugging. The VLSW 4150 supports SNMP protocol, RMON and Telnet functions for easy management. 3.2 Hardware Structure The hardware structure of the VLSW 4150 Layer 3 switch is divided into two parts: the processor module and the switching module. They are connected through the PCI interface and, together with the corresponding peripheral circuits, form a complete Layer 3 switch system, as shown in Figure 1. Figure 1 Hardware Structure (1) Processor Module As shown in Figure 2, the processor module uses a MOTOROLA PowerQUICC II CPU, which, together with some external storage devices and some peripheral circuits, constitutes the processor part of the Layer 3 switch. The processor module mainly runs the embedded operating system, configures the system and maintains the routing table, rather than being a component of the data forwarding path. The CPLD stores some CPU initialization configurations to ensure that the CPU starts normally after power-on. The Flash chip is used to store all the software and related configurations required by the Layer 3 switch. The SDRAM loads the program in the FLASH after the system starts to ensure the normal operation of the system. The processor module provides a Fast Ethernet interface and an asynchronous port for configuring and debugging the switch; on the other hand, it is connected to the switching module through the PCI interface, controls the switching module through the PCI interface, and performs data transmission [2]. Figure 2 Hardware composition of the processor module (2) Switching module As shown in Figure 3, the switching module uses the BCM5645 from BROADCOM as the ASIC chip. It communicates with the processor module through the PCI interface to complete data transmission. Through the memory interface provided by the 5645, a 64M external SDRAM can be provided to the switching module, thereby improving the throughput and switching speed of the switch. The 5645 connects 24 100M Ethernet and 2 Gigabit Ethernet through the MII interface and GMII interface respectively [3]. Figure 3 Hardware composition of the switching module 3.3 Software structure The software system of the VLSW 4150 three-layer switch adopts a modular and distributed design method, based on a real-time multi-tasking operating system. The software system has a hierarchical structure, with each layer built on top of another. Each layer uses the services provided by its neighboring layer below it and provides higher-level services to the layer above it. Its advantages are: it can shield the upper-layer software from the lower-level operations, improve the portability of the upper-layer software, and improve the maintainability of the software. As shown in Figure 4, the software is generally divided into three layers: (1) The driver layer connects the upper-layer software and the hardware system, and converts the routing update, management and configuration commands of the upper-layer software into a format that the hardware system can recognize, so as to update its internal data structure such as routing table, address table, etc., and control and manage the hardware switching system; at the same time, the device driver transmits the routing update messages, control management frames and various information received by the lower-layer hardware to the upper-layer software for processing; (2) The protocol stack implements protocols such as TCP/IP, 802.1D and 802.1Q, providing a good interface for the upper-layer application; (3) The application layer mainly includes the routing module and the network management module. The routing module implements protocols such as RIP and OSPF, that is, it implements the main functions of the third-layer routing; the network management module implements network management modules such as SNMP and RMON, so that the third-layer switch has some network management functions and ensures that the third-layer switch operates better. Figure 4. Software Structure 4. Application of Layer 3 Switches The main purpose of Layer 3 switches is to replace traditional routers as the core of a network. Therefore, wherever there is no need for wide area network connectivity but a router is still required, a Layer 3 switch can be used instead. In enterprise networks and campus networks, Layer 3 switches are generally used at the core layer of the network, using gigabit or 100 Mbps ports on the Layer 3 switch to connect different subnets or VLANs. Layer 3 switches solve the problem of LAN VLANs having to rely on routers for management, and resolve the network bottleneck problem caused by the low speed and complexity of traditional routers. Using Layer 3 switches to divide VLANs in a LAN can meet various flexible logical combinations of user terminals, prevent broadcast storms, and set different access permissions between different VLANs as needed, thereby increasing the overall network security, greatly improving the work efficiency of network administrators, and allowing for reasonable configuration of information resources, reducing network configuration costs, and making the connection between switches more flexible.