Layer 2 vs. Layer 3 Network Switches: Do you Know the Difference?

Network switches are essential devices that connect networks, and an IT professional must understand the role of switches in a properly functioning network. We often hear about Layer 2 and Layer 3 switches. This article will specifically discuss Layer 2 and Layer 3 switches in a network, what they are and their differences, and which one is better suited for building our network.

What is a Layer 2 Switch

Layer 2 switches work at Layer 2 (Data Link Layer) of the OSI model, hence called Layer 2 switches. The development of Layer 2 switching technology has become more mature. Layer 2 switches are data link layer devices that can identify MAC address information in packets, forward them according to the MAC address, and record these MAC addresses with the corresponding ports in an internal address table of their own. The specific workflow is as follows.

• When the switch receives a packet from a port, it first reads the source MAC address in the packet header to know on which port the machine with the source MAC address is connected.
• Then it goes and reads the destination MAC address in the packet header and looks up the corresponding port in the address table.
• If there is a port in the table corresponding to this destination MAC address, copy the packet directly to this port.
• If the corresponding port is not found in the table, the packet is broadcast to all ports. When the destination machine responds to the source machine, the switch can learn which port corresponds to the MAC address. It is no longer necessary to broadcast to all ports when transmitting data next time.

Continuously cycling this process, for the whole network, MAC address information can be learned, and this is how a Layer 2 switch builds and maintains its own address table.

Layer 2 switching technology has evolved from bridges to VLANs (Virtual Local Area Networks) and is widely used in LAN construction and transformation. Layer 2 switching technology works on the second layer of the seven-layer OSI network model, the data link layer. It forwards packets according to the destination MAC address of the received packets and is transparent to the network layer or higher-layer protocols. It does not deal with the IP address of the network layer or the port address of higher layer protocols such as TCP and UDP. It only needs the physical address of the packet, i.e., MAC address, and the data exchange is realized by hardware, and its speed is quite fast, which is a significant advantage of Layer 2 switching.

However, it cannot handle the exchange of data between different IP subnets. Traditional routers can handle many packets across IP subnets, but their forwarding efficiency is lower than that of Layer 2. Therefore, to take advantage of the high forwarding efficiency of Layer 2 and to handle Layer 3 IP packets, Layer 3 switching technology was born.

What is a Layer 3 switch?

A Layer 3 switch is a switch with some router functions. The most crucial purpose of a Layer 3 switch is to speed up the data exchange within a giant LAN, and the routing function it has serves this purpose by being able to route once and forward many times. For normal processes such as packet forwarding are realized by hardware at high speed. In contrast, the software recognizes functions like routing information update, routing table maintenance, route calculation, route determination, etc.

Layer 3 switching technology is Layer 2 switching technology + Layer 3 forwarding technology. Traditional switching technology operates at the second layer of the OSI network standard model – the data link layer. In contrast, the three-layer switching technology is the third layer in the network model to achieve high-speed forwarding of data packets, achieve network routing functions, and achieve optimal network performance according to different network conditions.

Network Backbone

It is not too much to say that Layer 3 switches in many network devices are “the mainstay.” In the campus network and metro education network, from the backbone, metro network backbone, convergence layer, there is a place for Layer 3 switches, especially the core backbone network must use Layer 3 switches. Otherwise, the entire network of thousands of computers in a subnet has no security and can not be divided into the broadcast domain and can not be the isolated broadcast storm.

If using a traditional router, it can isolate the broadcast, but the performance is not guaranteed. Layer 3 switches, on the other hand, offer very high performance, with the functionality of Layer 3 routing and the network speed of Layer 2 switching. While Layer 2 switching is based on MAC addressing, Layer 3 switching forwards service flows based on Layer 3 addresses; except for the necessary routing decision process, most of the data forwarding process is handled by Layer 2 switching the efficiency of packet forwarding.

The Layer 3 switch implements the routing function of IP by using a hardware switching mechanism. Its optimized routing software makes the routing process more efficient and solves the speed problem of traditional router software routing. Therefore, Layer 3 switches have the functionality of a router and the performance of a switch.

Connecting subnets

More than a certain number of computers on the same network (usually around 200, depending on the communication protocol) is likely to cause inefficient network transmission due to many broadcasts on the web. To avoid broadcast storms caused by broadcasts on large switches, they can be further divided into multiple virtual networks (VLANs). But this will lead to a problem: communication between VLANs must be done through routers. But traditional routers are also difficult to communicate between VLANs because the routing capability of traditional ordinary routers is too weak compared to the network traffic of LANs.

And the price of gigabit-class routers is also challenging to accept. Suppose different subnets or VLANs are connected using gigabit ports or 100-gigabit ports on a Layer 3 switch. In that case, it economically solves the problem of relying on routers for communication between subnets after subnetting while maintaining performance, so a Layer 3 switch is the ideal device for connecting subnets.

Difference between a Layer 3 switch and a Layer 2 switch

Layer 2 switches are for in small local area networks. In small LANs, broadcast packets have little impact. The fast switching capabilities, multiple access ports, and low, modest price of a Layer 2 switch provide a complete solution for small network users.

The advantages of a Layer 3 switch are its rich interface types, robust Layer 3 support, and routing capabilities, which make it suitable for routing between large networks.

The most crucial function of a Layer 3 switch is to speed up the fast-forwarding of data within an extensive local area network, and the addition of routing functions serves this purpose. Suppose a vast network is divided into small LANs according to departments, regions, and other factors. In that case, this will lead to a large number of inter-network access, and the simple use of Layer 2 switches can not achieve inter-network access. Such as the simple use of routers, due to the limited number of interfaces and slow routing and forwarding speed, will limit the speed of the network and network size, the use of fast-forwarding Layer 3 switches with routing function becomes the first choice.

Generally speaking, in the intranet data traffic, requiring fast-forwarding response network, such as all by the layer 3 switches to do this work, will cause the layer 3 switches to be overburdened, the response speed is affected, the inter-network routing to the router to complete, give full play to the advantages of different devices, is not a good networking strategy, of course, provided that the customer has enough money budget. Otherwise, it lets the three-layer switch also for the inter-network interconnection.

To sum up, there is still a vast difference between a Layer 3 switch and a Layer 2 switch. No matter from which aspect, it is better to use a layer 3 switch for multi-subnet connection in LAN, especially in the environment where different subnets exchange data frequently. On the one hand, it can ensure the communication performance demand between subnets, and on the other hand, it saves the investment of purchasing another switch. OPTCORE provides a series of SFP transceivers, SFP+ transceivers, QSFP+ transceivers, and QSFP28 transceivers suitable for Layer2/3 Ethernet network switches.