Bridges and Layer2 Switches

Bridges operate in the data-link layer of the OSI model. Bridges learn the MAC layer addresses of each node of the segments and remember off which port the MAC addresses are located. The bridge builds a table of MAC addresses and ports. If the destination MAC address of an incoming frame is not in the table, bridges forward the frame to all ports (minus the port from which the frame came). If the destination MAC address is in the table, bridges forward the frame only if the destination MAC address is on another port. Bridges filter the frame if the destination MAC address is located on the same port on which the frame arrived.

Bridges are store-and-forward devices. They store the entire incoming frame and verify the checksum before forwarding the frame. If a checksum error is detected, the frame is discarded.

Figure 2-6 shows that bridges define the collision domains; each port off a bridge is a separate collision domain. Collision domains are also referred to as bandwidth domains because all devices in the collision domain share the same bandwidth. Bridges do not control broadcasts. Bridges flood broadcasts out all ports. Bridges are protocol transparent; they are not aware of upper-layer protocols, such as IP and IPX. Bridges are designed to flood all unknown and broadcast traffic.

Figure 2-6 Bridges Control Collision Domains

Bridges

Bridges

■<-Broadcast Domain->.

Switches use fast integrated circuits that reduce the latency common to regular bridges. Switches are the evolution of bridges. Some switches have the capability to run in cut-through mode where the switch does not wait for the entire frame to enter its buffer; instead, it begins to forward the frame as soon as it finishes reading the destination MAC address. Cut-through operation increases the probability that error frames are propagated on the network because the frame is forwarded before the entire frame is buffered and checked for errors. Because of these problems, most switches today perform store-and-forward operations as bridges do. Switches are exactly the same as bridges with respect to collision domain and broadcast domain characteristics. Each port on a switch is a separate collision domain. All ports in a switch are in the same broadcast domain.

Various types of bridges exist, including transparent, translational, source-route, and so on. These bridging types are covered in Chapter 4.

Continue reading here: Routers and Layer3 Switches

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