Bridges / Switches

Bridges
5.1.9 This page will explain the function of bridges in a LAN.


There are times when it is necessary to break up a large LAN into smaller and more easily managed segments. This decreases the amount of traffic on a single LAN and can extend the geographical area past what a single LAN can support. The devices that are used to connect network segments together include bridges, switches, routers, and gateways. Switches and bridges operate at the data link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.

When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to determine whether to filter, flood, or copy the frame onto another segment. This decision process occurs as follows:

• If the destination device is on the same segment as the frame, the bridge will not send the frame onto other segments. This process is known as filtering.

• If the destination device is on a different segment, the bridge forwards the frame to the appropriate segment.

• If the destination address is unknown to the bridge, the bridge forwards the frame to all segments except the one on which it was received. This process is known as flooding. If placed strategically, a bridge can greatly improve network performance.

The next page will describe switches.

Switches
5.1.10 This page will explain the function of switches.


A switch is sometimes described as a multiport bridge. A typical bridge may have only two ports that link two network segments. A switch can have multiple ports based on the number of network segments that need to be linked. Like bridges, switches learn information about the data packets that are received from computers on the network. Switches use this information to build tables to determine the destination of data that is sent between computers on the network.

Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. A bridge determines whether the frame should be forwarded to the other network segment based on the destination MAC address. A switch has many ports with many network segments connected to them. A switch chooses the port to which the destination device or workstation is connected. Ethernet switches are popular connectivity solutions because they improve network speed, bandwidth, and performance.

Switching is a technology that alleviates congestion in Ethernet LANs. Switches reduce traffic and increase bandwidth. Switches can easily replace hubs because switches work with the cable infrastructures that are already in place. This improves performance with minimal changes to a network.

All switching equipment perform two basic operations. The first operation is called switching data frames. This is the process by which a frame is received on an input medium and then transmitted to an output medium. The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops.

Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.

An Ethernet switch has many benefits. One benefit is that it allows many users to communicate at the same time through the use of virtual circuits and dedicated network segments in a virtually collision-free environment. This maximizes the bandwidth available on the shared medium. Another benefit is that a switched LAN environment is very cost effective since the hardware and cables in place can be reused.

The Lab activity will help students understand the price of a LAN switch.