How does a network switch work


A switch is a coupling element that connects multiple hosts in a network. In an Ethernet network based on the star topology, a switch serves as a distributor for the data packets.
The function is similar to a hub, with the difference that a switch can switch direct connections between the connected devices, provided it knows the ports of the data packet receivers. If not, the switch broadcasts the data packets to all ports. When the response packets come back from the recipients, the switch notes the MAC addresses of the data packets and the associated port and then only sends the data packets there.
While a hub limits the bandwidth of the network, the connection between two hosts has the full bandwidth of the end-to-end network connection available.

A switch works on the data link layer (layer 2) of the OSI model and works in a similar way to a bridge. This is why such terms have become established among manufacturers, such as: B. Bridging Switch or Switching Bridge. However, they are no longer used today.

Switches are distinguished in terms of their performance with the following properties:

  • Number of storable MAC addresses for the source and destination ports
  • Procedure when a received data packet is forwarded (switching procedure)
  • Latency (delay time) of the transmitted data packets

In principle, a switch is nothing more than an intelligent hub that remembers which host can be reached via which port. In this way, each switch port creates its own collision domain.
Expensive switches can also work on layer 3, the network layer, of the OSI layer model (layer 3 switch or layer 3 switch). You are able to forward the data packets to the destination ports based on the IP address. In contrast to normal switches, logical delimitations can also be achieved without a router.


Switching is a mechanism in packet-oriented networks to determine the correct exit for incoming data packets. The aim is to switch a connection between an input port and an output port on the basis of sender and receiver addresses.

Collision Domain

The CSMA / CD method causes collisions when several hosts are connected to a collision domain. This in turn reduces the network traffic caused by repetitive transmissions. Setting up multiple collision domains reduces the number of data packet collisions.
Switches form a collision domain on each of their ports by only forwarding the data traffic to the port on which the destination MAC address is located. Within a collision domain (switch port) there is usually a single host, another switch or a router to another network.

MAC address management / MAC table

In contrast to hubs, switches have the advantage that they only forward data packets to the port to which the host with the destination address is connected. The MAC address, i.e. the hardware address of a network card, serves as the assignment. The switch saves this address in an internal table. If a switch receives a data packet, it searches for the port in its memory under the destination address (MAC) and then only sends the data packet to this port. A switch gets to know the allocation of the MAC addresses over time. The number of addresses a switch can hold depends on its internal memory.
A quality feature of a switch is how many addresses it can store in total and per port. On a switch that only connects a handful of computers, it doesn't matter how many addresses it can manage. However, if the switch is in a large network and other switches and hubs are connected to its ports, then it may need to be able to save several thousand MAC addresses and assign them to the ports. The larger a network, the more important it is to ensure that the switches have sufficient capacity for managing MAC addresses.

Additional features of switches


The data in a switch are transmitted via what is known as the backplane. All ports that have to exchange data with each other are connected via the backplane. The bandwidth must therefore be large enough to serve all connected stations at the network speed.
The following rule of thumb applies: The bandwidth of the backplane must be twice the maximum possible bandwidth of all available ports. In simplified terms, the following formula applies:

The result must be doubled again (x2) if the data is transmitted with full duplex.
A switch with 5 Fast Ethernet connections (100 Mbit / s) therefore requires a backplane bandwidth of 1 Gbit / s (1000 Mbit / s).

In small network installations, the backplane bandwidth is usually irrelevant. Even cheap switches have a sufficiently large backplane bandwidth. However, if the switch serves as a distributor between stations with database queries and file transfers, then the bandwidth plays a greater role.
Simpler switches are usually based on a bus architecture with a high bandwidth, since the circuit complexity is lower and the device can be manufactured more cheaply.

Problem case: Switch

If the data traffic in a network only flows to a single host, the switch has little influence on the performance. A switch can temporarily store a small number of data packets. At some point it discards the incoming data packets or creates collisions.
Discarded data packets are sent again by higher-level protocols such as TCP or IPX. With unsecured protocols such as B. UDP or NetBIOS can lead to connection interruptions. Collisions are recognized on layer 2 and requested again. In any case, there are noticeable delays in the network traffic.


There are switches in a wide variety of designs and expansion stages. In general, the bigger and better equipped a switch, the more expensive it is. A simple switch has 4 or 5 ports. With a couple of status LEDs, they are built into boxes the size of a cigarette packet. Slightly better switches have a metal housing, are more stable and better suited for continuous use. In addition to the small 4- and 5-port switches, there are expansion stages with 8, 16, 24 and 32 ports. If you need more ports, you need stackable switches that can be connected to each other via separate cables and stacked on top of each other. If you want to install switches in 19 "cabinets, you should pay attention to 16-, 24- and 32-port switches. These have brackets for 19" mounting rails. The small devices sometimes have brackets on the bottom and can be mounted on the wall.
If you buy small switches with a small number of ports and do not install them in a 19 "cabinet, you should pay attention to the arrangement of RJ45 sockets and status LEDs. There are devices with the LEDs on the front and the connections on the back In cheap devices, the status LEDs are integrated into the RJ45 sockets. This is not always practical, eg if the cables do not come from the direction where you want the status LEDs to be clearly visible. Small switches are usually used powered by a plug-in power supply and have no on and off switch. Devices with a larger number of ports have an integrated power supply and a fan for cooling. Be careful when using in private areas. The fan noise should not be underestimated.

Managed Switch / Enterprise Switch

A managed switch has additional equipment features and performance features that can be configured. Furthermore, such a switch can also be managed remotely centrally, which is particularly important in the enterprise environment.

Software Defined Networking (SDN)

Software-defined networking, software-defined network or software-defined network, or SDN for short, includes network components whose functions can be programmed individually. A higher-level control of all network components is possible at the same time.

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