segment, it transmits the packet on that segment or port to that segment only. If the bridge does not know the
destination segment, the bridge transmits a copy of the frame to all the interface ports in the source segment
using a technique known as flooding. For each packet an interface receives, the bridge stores in its table the
following information:
The frame’s source address• The interface the frame arrived on• The time at which the switch port received the source address and entered it into the switching table•
Note Bridges and switches are logically equivalent.
There are four kinds of bridges:
Transparent bridge—Primarily used in Ethernet environments. They are called transparent bridges
because their presence and operation are transparent to network hosts. Transparent bridges learn and
forward packets in the manner described earlier.
•
Source−route bridge—Primarily used in Token Ring environments. They are called source−route
bridges because they assume that the complete source−to−destination route is placed in frames sent
by the source.
•
Translational bridge—Translators between different media types, such as Token Ring and Ethernet.• Source−route transparent bridge—A combination of transparent bridging and source−route bridging
that enables communication in mixed Ethernet and Token Ring environments.
•
Broadcasts are the biggest problem with bridges. Some bridges help reduce network traffic by filtering
packets and allowing them to be forwarded only if needed. Bridges also forward broadcasts to devices on all
segments of the network. As networks grow, so does broadcast traffic. Instead of frames being broadcast
through a limited number of devices, bridges often allow hundreds of devices on multiple segments to
broadcast data to all the devices. As a result, all devices on all segments of the network are now processing
data intended for one device. Excessive broadcasts reduce the amount of bandwidth available to end users.
This situation causes bandwidth problems called network broadcast storms. Broadcast storms occur when
broadcasts throughout the LAN use up all available bandwidth, thus grinding the network to a halt.
Network performance is most often affected by three types of broadcast traffic: inquiries about the availability
of a device, advertisements for a component’s status on the network, and inquiries from one device trying to
locate another device. The following are the typical types of network broadcasts:
Address Resolution Protocol (ARP)• Internetwork Packet Exchange (IPX) Get Nearest Server (GNS) requests• IPX Service Advertising Protocol (SAP)• Multicast traffic broadcasts• NetBIOS name requests•
These broadcasts are built into the network protocols and are essential to the operation of the network devices
using these protocols.
Due to the overhead involved in forwarding packets, bridges also introduce a delay in forwarding traffic. This
delay is known as latency. Latency delay is measured from the moment a packet enters the input port on the
switch until the time the bridge forwards the packet out the exit port. Bridges can introduce 20 to 30 percent
loss of throughput for some applications. Latency is a big problem with some timing−dependent technologies,
such as mainframe connectivity, video, or voice.
High levels of latency can result in loss of connections and noticeable video and voice degradation. The
inherent problems of bridging over multiple segments including those of different LAN types with Layer 2
devices became a problem to network administrators. To overcome these issues, a device called a router,
operating at OSI Layer 3, was introduced.
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