A network switch is a fundamental piece of any network, so it’s critical that you as an IT professional understand the role of a switch in a properly functioning network. And to best understand the difference between Layer 2 switches and Layer 3 switches, you also need to know the difference between Layer 2 and Layer 3 in the OSI networking model. Show Layer 2 vs Layer 3 switchesThe OSI networking model defines a number of network “layers.” (Getting into each layer is beyond the scope of this article but our Network Management in a Nutshell blog post has a good recap if you want to brush up.) Layer 2 of the OSI model is known as the data link layer. The Layer 2 protocol you’re likely most familiar with is Ethernet. Devices in an Ethernet network are identified by a MAC (media access control) address, which is generally hardcoded to a particular device and doesn’t normally change. Layer 3 is the network layer and its protocol is the Internet Protocol or IP. Devices in an IP network are identified by an IP address, which can be dynamically assigned and may change over time. Traditionally, the network device most associated with Layer 3 has been the router, which allows you to connect devices to different IP networks. Network switches definedSwitches are one of the traffic directors on the network, and traditionally operate at Layer 2. They allow for the connection of multiple devices in a LAN while decreasing the collision domain by employing packet switching. By inspecting the contents of packet headers, a switch builds up a table of MAC addresses and their corresponding physical ports on the switch to intelligently make decisions on directing future packets. Then, when a packet arrives at the switch, the switch inspects the header of the packet to determine the destination, consults the table of MAC addresses with their corresponding physical ports, and makes a decision on which physical port to send the packet out to. Switches can get a bit more complicated when you introduce VLANs (virtual LANs). VLANs allow you to carve off components of one physical device into different networks, essentially separating one network of physically connected devices into multiple logical networks that can’t directly communicate with one another. VLANs support one of the tenets of good network design: network segmentation. There’s a bit more to how a switch works, but that should cover the basics. Bringing it all togetherFor two devices to communicate across a typical business or home network, they need to have both an IP address, associated with Layer 3 (the IP layer), and a MAC address, associated with Layer 2 (the Ethernet layer). In legacy networks, built before there were smart switches capable of supporting VLANs, the only way for two devices on separate Layer 2 Ethernet networks was to be routed between those two networks. The routing was done by a Layer 3 device called… a router. As network technologies progressed and VLANs were introduced, managed switches gained the ability to connect two devices on separate Ethernet networks. While this reduced the need to have different physical switches for each Ethernet network, devices connected to two separate VLANs still needed to communicate through a Layer 3 device, which in most networks was a router. Then came the Layer 3 switch. This device operates at both Layer 2 and Layer 3, allowing devices connected to different VLANs to communicate with one another without going through a dedicated router. It’s important to note the traffic is still being routed, as this is the terminology we use to describe information transferred between networks at Layer 3. The routing is simply being done by the switch instead of a dedicated router. So does this mean that all Layer 3 switches do routing? Not exactly. Just because a device is Layer 3-capable, doesn’t necessarily mean the device is performing routing. As a network administrator, you need to configure the device to route traffic between VLANs if that’s what you want. You can have a Layer 3-capable switch operating in Layer 2-only mode. With the functionality of most managed switches today, having your switch act as a Layer 3 device is an option on all but most entry-level switches. So what happens when a Layer 3 switch receives a packet from an end device? When inspecting the packet header, if that packet is destined for another VLAN, the Layer 3 switch “elevates” the packet to the routing layer. A decision is then made at the Layer 3 routing layer on where to send the packet—the switch consults the MAC address forwarding table to decide which port to send the outgoing packet on. And there you have it: a switch that makes routing decisions on traffic and therefore operates at Layer 3. When should you use Layer 3 switches?The recommendation on whether to use a switch at Layer 2 or Layer 3 depends in part on the size and complexity, and security requirements, of the network you’re managing. When designing your network topology, consider some of the following points:
Pros and cons of Layer 3 switchesWhy would you choose to use a Layer 3 switch? What are the pros and cons? Pros
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What about Layer 3 routers?With all this talk about Layer 3 switches, are dedicated routers a thing of the past? In most small to midsize networks, a dedicated router for intra-office communication is no longer required. For example, if you put your users on a separate VLAN from your network infrastructure like servers, then the routing of traffic between the users and the server can be done on either a Layer 3 switch or a firewall. No need for a dedicated router. But routers do still have an important role to play in many business networks, especially for communication outside of the local network. Connecting to remote offices or the internet requires connecting to a non-Ethernet network, like that provided by your ISP, and this is where routers shine. Routers can also be found in larger enterprise networks where routers are often still dedicated devices. —
Routers and switches are both computer networking devices that allow one or more computers to be connected to other computers, networked devices, or to other networks. The functions of a router, switch and hub and are all different, even if at times they are integrated into a single device. Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term layer 3 switch often is used interchangeably with router, but switch is really a general term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types.
A router is a networking device that connects computer networks, for example, connecting a home network with the Internet. Routers are the workhorses that transfer packets of data between networks to establish and sustain communication between two nodes in an internetwork. Routers operate at Layer 3 (network layer) of the OSI model; a router uses the destination IP address in a data packet to determine where to forward the packet. What is a Network Switch?A network switch connects devices together on a single computer network. A switch is also called switching hub, bridging hub, or MAC bridge. Switches use MAC addresses to forward data to the correct destination. A switch is considered a Layer 2 device, operating at the data link layer; switches use packet switching to receive, process and forward data. What is a Network Hub?Network hubs — also called repeaters — are even less advanced that switches; while a hub broadcasts the same data to all its ports, a network switch forwards data only to those devices that the data is intended for. Network hubs do not manage any traffic coming through them; they only broadcast — or repeat — packets from an incoming port to all other ports. The following video compares hubs, switches, and routers. Function of a Switch vs. a RouterA router is a more sophisticated device than a switch. Traditional routers are designed to join multiple area networks (LANs and WANs). Routers serve as intermediate destinations for network traffic. They receive TCP/IP packets, look inside each packet to identify the source and target IP addresses, then forward these packets as needed to ensure the data reaches its final destination. In addition, routers often perform network address translation (NAT), which allows all devices on a subnetwork (e.g., all devices in a home) to share the same public IP address. Finally, routers that include built-in firewalls improve the network's security. A network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Switches are incapable of joining multiple networks or sharing an Internet connection. A home network with a switch must designate one computer as the gateway to the Internet, and that device must possess two network adapters for sharing, one for the home LAN and one for the Internet WAN. With a router, all home computers connect to the router equally, and it performs the equivalent gateway functions. ConnectivityRouters can connect wired or wireless (WiFi) networks. A switch is used for wired networking connections. IntelligenceRouters are more sophisticated devices that can have software to increase network throughput using techniques such as caching. References
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