Network Bridge Concepts

The Concept of a Network Bridge

Network Bridge is the basic concept of today LAN Switches. Understanding the Switches, you should understand the basic concept of the network bridge. Today, the function of Network Bridge in wireless computer networking is commonly integrated into the wireless access point.

Network Bridge forwards LAN broadcast frames, and unknown unicast frames, out all ports. LAN broadcasts, by definition, are received by all devices on the same LAN. So the switch simply forwards broadcasts out all ports, except the one on which the broadcast was received. Switches forward unknown unicast frames, which are frames whose destination MAC addresses are not yet in the bridging table, out all ports as well. The switch floods the frame with the hope that the unknown device will be on some other Ethernet segment, it will reply, and the switch will build a correct entry in the address table.

Generally speaking, switches also forward LAN multicast frames out all ports, just like they do for broadcasts. However, a few multicast features in switches limit the flooding of multicasts, such as Internet Group Management Protocol (IGMP) snooping. The following section will discuss more detail about Network Bridge.

Since the original concept of the Network Bridge is used for Switches today, understanding more detail about bridge is important including Bridges characteristic, advantages, and limitation.

Bridges have the following characteristic:

Bridges Connect two media segments that use the same protocol
Bridges examine the source address to determine the media segment of network devices
Bridges operate at the data link layer of the OSI model
Bridges maintain a table of devices address and their corresponding segments
Each segment connected by a bridge can have the same network address
Messages within a media segment are prevented from crossing over to another segment

The Advantages of Network Bridge

Switches (which use the Network Bridge concept) in today industry replace the legacy Hubs devices to increase the performance of the LAN.

Bridges prevent wasted bandwidth by eliminating unnecessary traffic between segments
Bridges increase the maximum network length
Bridges forward packets for multiple upper-layer protocols
Bridges can link segments with dissimilar transmission media and media access methods

The Limitation Architecture of The Network Bridge

Besides the advantages, Network Bridges have the architecture limitation.

Bridges cannot link multiple architecture because different frame types are used
Bridges cannot translate upper-layer protocols
Bridges cannot forward packets to different networks based on the network address. Networking routers are used for internetworking communications.
Bridges do not filter broadcast packets

You can use Network Bridge to isolate traffic to a segment, or to prevent unwanted traffic from crossing over to other segments, or to slow WAN links. When designing the placement of network bridges on the network, follow the 80/20 rule.

At least 80% of the network traffic should stay within a segment
No more than 20% of network traffic should pass through the bridge to another segment

Because most network traffic is local to a particular segment, devices on opposite sides of the bridge can transmit data simultaneously without collisions. This increases the network bandwidth.

There are three types of Network Bridge:

Transparent bridges (for Ethernet and Token ring networks)
Source-routing bridges (for Token ring networks)
Source-routing transparent bridges (for Token ring networks)

Transparent Bridges

When transparent bridges first were introduced, Ethernet networks were either 10BASE5, 10BASE2, or 10BASE-T. Each of these three types of Ethernet had some common characteristics that drove the need for a Network Bridge device:

Any device sending a frame could have the frame collide with a frame sent by any other device attached to that LAN segment.
Only one device could send a frame at a time, so the devices were sharing the 10-Mbps bandwidth.
Broadcasts sent by one device would be heard by all other devices on the LAN.

When these three types of Ethernet first were introduced, a shared 10-Mbps of bandwidth was a huge amount of bandwidth! Before the introduction of LANs, people often used dumb terminals, with a 56-kbps WAN link being a really fast connection to the rest of the network—with that 56-kbps being shared among everyone in the building. So, getting to put your computer on a 10BASE-T Ethernet LAN was like getting a Gigabit Ethernet connection for your PC at your desk at work today—it was more bandwidth than you could imagine that you would need.

A Network Bridge is transparent when the sending and receiving devices are unaware of the presence of the bridge. As far as both devices are concerned, they are on the same network segment. Transparent bridges maintain a database of all network devices and their corresponding segment. The bridge database can be created in either of the following ways:

Manually – a network administrator uses special software to make device/segment entries in the bridge database
Automatically – transparent bridges can learn the network configuration. The database is created as messages are sent from one device to another.

Transparent bridges are also called learning bridges because of their ability to learn and remember the network configuration. Transparent bridges build the database by reading the source address in incoming packets.

Transparent bridge summary:

The following facts should be kept in mind about how transparent bridges work.

Transparent bridges build the device/segment database by examining the source address of incoming packets.
Transparent bridges forward packets based on the following rules:
If the destination address is unknown, the bridge forwards the packet to all other segments
If the destination address is known and on the same segment, the bridge discards the packets
If the destination address is known and on different segment, the bridge forwards the packets to the appropriate segment.
Transparent bridges forward packets only if the following conditions are met.
The frame contains upper-layer data (data from the LLC sublayer on up)
The frame’s integrity has been verified (a valid CRC)
The frame is not addressed to the bridge

By understanding the concept of the network bridge, you will easily understand the concept of Spanning tree protocols which are the essential protocols in the implementation of large networking environments.