To ensure security and facilitate management, large local area networks (LANs) need to be divided into smaller LANs based on functional or geographical factors, reducing the impact of broadcast storms. VLAN technology has become widely used in such networks, where communication between different VLANs requires routing. However, relying solely on routers for inter-network access has limitations in terms of scalability, port availability, and routing speed. To address these issues, layer 3 switches were introduced.
Layer 3 switches are specifically designed for IP networks. They feature simplified interface types, robust second-layer packet processing capabilities, and are highly suitable for data routing and switching in large LANs. They can operate at Protocol 3, replacing or partially performing the functions of traditional routers, while maintaining comparable speed to second-layer switches. Additionally, layer 3 switches offer cost-effective solutions for network infrastructure.
In enterprise and educational networks, layer 3 switches are commonly deployed at the core layer, where they utilize gigabit or megabit ports to connect different subnets or VLANs. However, it is important to note that the primary purpose of layer 3 switches is to enhance data exchange within large local area networks, and their routing capabilities are designed with this objective in mind. As a result, their routing functions may not be as robust as those of dedicated routers at the same level. Layer 3 switches have certain limitations, particularly in areas such as security and protocol support. Therefore, they cannot completely replace the functionality of routers.
If we want to describe the role of the layer 3 switch among many network devices, "the cornerstone" is not an exaggeration. In campus networks and metropolitan education networks, layer 3 switches have their place in the backbone network, metropolitan area network backbone, and convergence layers, especially in the core backbone network. If we don't use layer 3 switches, tens of thousands of computers in the entire network will be in the same subnet, not only is it not secure, but it will also be unable to isolate broadcast storms because we cannot divide the broadcast domain.
When using traditional routers, although broadcast isolation is possible, performance cannot be guaranteed. On the other hand, layer 3 switches offer exceptional performance by combining the routing functionality of the third layer with the network speed of the second layer. Second-layer switching operates based on MAC addressing, while third-layer exchange is carried out by forwarding business flows using third-layer addresses. With the exception of necessary routing decision-making processes, most data forwarding tasks are handled by second-layer switching, resulting in improved data packet forwarding efficiency.
Layer 3 switches implement IP routing functions through hardware switching structures, and their optimized routing software enhances the efficiency of the routing process, addressing the speed issues commonly associated with traditional router software routing. Therefore, it can be concluded that layer 3 switches offer both "routing functions and switching performance".