In the realm of networking, Layer 2 switches have traditionally served as fundamental components for connecting network and client devices. However, with the ever-increasing variety of network applications and the adoption of converged network setups, Layer 3 switches have gained prominence in data centers, intricate enterprise networks, and commercial environments. This leads us to ponder: when it comes to network switches, which option should be favored – Layer 2 or Layer 3?
What are L2 and L3 Switch?
In the context of network communications, Layer 2 and Layer 3 switches find their application within the Open Systems Interconnection (OSI) model. The OSI model serves as a reference framework that facilitates the understanding and explanation of network protocols. Consisting of seven layers, namely the application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer, Layer 2 and Layer 3 pertain to the data link layer and network layer, respectively. Therefore, the switches operating at these layers are referred to as Layer 2 switches and Layer 3 switches.
L2 vs L3 Switch
The main difference between Layer 2 and Layer 3 switches lies in their routing capabilities. A Layer 2 switch operates solely based on MAC addresses and does not consider IP addresses or higher-layer information. On the other hand, a Layer 3 switch, also known as a multilayer switch, can perform all the functions of a Layer 2 switch while also providing additional static and dynamic routing capabilities. This means that a Layer 3 switch maintains both a MAC address table and an IP routing table, allowing it to handle intra-VLAN communication as well as routing packets between different VLANs.
There is also a category known as Layer 2+ (or Layer 3 Lite) switches, which offer static routing as an additional feature. Layer 3 switches go beyond packet routing and include functions that require an understanding of IP address information, such as VLAN traffic tagging based on IP addresses instead of manual port configuration. Layer 3 switches have gained popularity due to their increased power and security features, catering to evolving network demands.
When deciding between Layer 2 and Layer 3 switches, it is important to consider the intended usage scenario. In a pure Layer 2 domain, where hosts are directly connected, a Layer 2 switch is sufficient and recommended. This is typically referred to as the access layer in network topology. However, if there is a need to aggregate multiple access switches and perform inter-VLAN routing, a Layer 3 switch becomes necessary. This scenario is commonly referred to as the distribution layer in network topology.
Key Parameters to Consider When Purchasing
When considering the purchase of a Layer 2 or Layer 3 switch, there are several key parameters that you should examine, including the forwarding rate, backplane bandwidth, number of VLANs, MAC address table size, and latency.
The forwarding rate, also known as the throughput rate, refers to the switch’s ability to forward packets. If the forwarding capabilities exceed the combined speeds of all ports, the backplane is considered non-blocking. The forwarding rate is typically measured in packets per second (pps). You can calculate the forwarding rate of a switch using the following formula:
Forwarding Rate (pps) = (Number of 10 Gbit/s ports * 14,880,950 pps) + (Number of 1 Gbit/s ports * 1,488,095 pps) + (Number of 100 Mbit/s ports * 148,809 pps)
For example, let’s consider the FS S5850-32S2Q, which has 32 10 Gbit/s ports and 2 40 Gbit/s ports. Its forwarding rate would be:
32 * 14,880,950 pps + 2 * 4 * 14,880,950 pps = 595,238,000 pps ≈ 596 Mpps
The next parameter to consider is the backplane bandwidth or switch fabric capacity, which represents the cumulative speed of all ports. The speed of each port is counted twice, once for transmit (Tx) direction and once for receive (Rx) direction. Backplane bandwidth is typically expressed in bits per second (bps).
Backplane Bandwidth (bps) = Port number * Port data rate * 2
Using the example of the S5850-32S2Q, the backplane bandwidth would be:
(32 * 10 Gbps + 2 * 40 Gbps) * 2 = 800 Gbps
Other important parameters include the number of VLANs that can be configured. For a Layer 2 switch, a typical value is 1K (1024 VLANs), while a Layer 3 switch typically supports 4k (4096) VLANs. The memory of the MAC address table refers to the number of MAC addresses that a switch can store, often expressed as 8k or 128k. Latency represents the delay that occurs during data transfer and should ideally be minimized. Latency is usually expressed in nanoseconds (ns).
By considering these parameters, you can make an informed decision when selecting a Layer 2 or Layer 3 switch that meets your specific network requirements.
Summary
This article delves into the disparities between Layer 2 and Layer 3 switches, outlining their respective functionalities to aid in decision-making between the two. Furthermore, crucial metrics for evaluating these switches are explored. It is important to note that a more advanced device may not always be superior; instead, selecting the most suitable option for your specific application is paramount.
