Network bridge (Layer 2 device)

There is no clear classification of technology in the technical literature that defines bridges or switches. Switches work as transparent bridges, but have a higher throughput performance and more ports. In addition, modern switches are also often equipped with a Layer 3 instance, a simple router. In general, bridges were developed and marketed around 1985 for segmenting (reducing the collision domain) networks and connecting different architectures (e.g. Ethernet - TokenRing). Switches were developed much later (1990). They can replace routers in some circumstances, even when they do not contain their own Layer 3 instance. For example, when the use of a switch instead of a bridge became necessary to reduce a collision domain and a bridge did not have enough ports and throughput.

To reduce the collision domain, a switch is given as many ports as possible to which only a few devices - ideally one - are connected at a time. In addition, one or more so-called uplink ports establish connections to the next switch or router. Often, but not necessarily, uplink ports are implemented in a faster or higher-quality (Ethernet) technology than the other ports (e.g. Gigabit Ethernet instead of Fast Ethernet or fiber optic cable instead of twisted-pair copper cable). Non-modular switches typically have a minimum of four ports to a maximum of about 48 ports. Large "modular" switches can be configured into units with several hundred ports, depending on the model. Unlike bridges, switches can transmit multiple packets simultaneously between different pairs of ports. A bridge corresponds most closely to a switch in the operating mode Store and Forward with usually only two ports: a switch is a multiport bridge was still a slogan of Cisco Systems in 1991; since the takeover of Kalpana in 1994, Cisco has taken a more differentiated approach to the topic.

In the early days of switch technology, port switches were also widespread. These were less expensive devices that had a dedicated uplink port and could only store one MAC address per port on the remaining ports. Bridges, on the other hand, can always store many MAC addresses in their internal SAT (Source Address Table). Conversely, bridges often require external distributors, such as hubs, to connect multiple devices.

As a rule, bridges and switches can connect networks with different transmission speeds. Bridges can usually operate on both a MAC and LLC basis, whereas switches operate on a MAC basis. Consequently, switches cannot bridge different architectures (e.g. Ethernet - Token Ring). Since Ethernet dominates the market, bridging different LAN architectures is of little importance. Not least for this reason, bridges are now niche products.

With larger switches, just as with powerful bridges, a certain bandwidth can be defined for each connected network segment, and certain services can also be prioritized (quality of service). In addition, large modern switches support a wide variety of protocols and procedures (e.g. Discovery protocols, VLANs, MANs, QoS, Layer 3 instance with various routing protocols, management protocols (SNMP, RMON, Syslog), infrastructure protocols (DHCP server, BOOTP/TFTP server, FTP server, SSH server), special handling for special protocols (DHCP and BOOTP relay agent), security features (Layer 2 to 4 ACLs, Gratuitous ARP Protection, DHCP Enforcement, MAC Lockdown, Broadcasting Control, Ingress Filter), redundancy protocols (VRRP), etc.). In the process, the differences to routers are also becoming more and more blurred.

Bridges, which are set up within an operating system, play a major role in virtualization. Here, a so-called bridge device is set up, which extends a real network card with virtual network cards and connects them like a bridge. These interfaces are made available to the virtualized guest system as (virtual) network cards. Only via these network cards is the external network communication of a guest system via the real network interface of the host system also possible to the outside.