Transparent Bridging

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While routers interconnect networks, it is usually the job of switches and bridges to interconnect the various network segments (subnets), network assets and network devices (printers, Internet connection, storage and servers etc.) along with user nodes (workstations). The manner in which this is done will be dependent upon such criteria as: network architecture, transmission media, topology (physical and logical) and current network infrastructure, assets and devices.

Bridging Types

While Ethernet networks use transparent bridging other network architectures use other bridging technologies. For example; IBM’s Token Ring networks use a type of bridging known as source-route bridging.

Transparent Bridges/Switches – The reason that transparent bridges (including modern switches) are so named is because their presence and operations are to all intents and purposes transparent (not readily visible) to network hosts and users alike.

Hybrid Bridging Networks – Note that Ethernet and Token Ring are incompatible and cannot be used on the same network segment. However; it is possible for a Local Area Network (LAN) to be composed of a number of subnet segments, each of which may use either transparent bridging or source-route bridging. Networks of this type (multiple bridging technologies) use routers to interconnect the various segments.

Learning, Forwarding, Filtering and Loop Avoidance

It is these three functions (learning, forwarding and filtering) that determine how a transparent bridge decides whether or not it should forward a frame and onto which interface it should place that frame. Another function critical to the effective, efficient, trouble-free functionality of transparent bridges is loop avoidance. Redundancy and loop avoidance issues are covered in another article entitled “Loop Avoidance”.


When a transparent bridging device is rebooted or powered up for the first time it immediately begins to learn the location of network devices by analyzing the source Media Access Control (MAC) Address of all incoming frames from all attached networks.

Filter Table Building

New Source MAC Address – Upon examining an incoming frame and learning its source MAC Address the transparent bridge will check its Filter Table to see if it has an entry pertaining to that MAC Address. If there is no existing entry corresponding to the incoming frame’s source MAC Address the transparent bridging device will create a new entry in its Filter Table that associates this MAC Address as being reachable via the port it just arrived on.

As more traffic transpires over time the transparent bridge will continue building its Filter Table. New entries are created for each new (learned) source MAC Address along with the corresponding port through which this address can be reached.

Existing MAC Address Entries – If there is already an entry in the transparent bridge’s Filter Table pertaining to this MAC Address the transparent bridge will compare the existing MAC Address details with those of the incoming frame. If they both match the transparent bridge moves onto the next phase.

If the existing and current MAC Address details conflict the transparent bridge will overwrite the old entry with the new MAC Address/ port details. Every time a frame arrives the transparent bridging device will repeat this process. It is in this way that a transparent bridging device builds its Filter Table.

Forwarding – Flooding

Transparent bridging devices also examine incoming frames to determine their intended destination Media Access Control (MAC) Address. Once the transparent bridging device has determined the intended destination MAC Address to which this frame should be sent it refers to its Filter Table to check for any entries relating to this MAC Address. If; there is no information identifying which port this MAC Address can be reached via, the transparent bridging device will simply “flood” the frame out every port except the one on which it arrived.

Forwarding – Filtering

Eventually the transparent bridging device will receive an incoming frame whose destination MAC Address is contained in the transparent bridging device’s MAC Address Filter Table. There will also be an entry telling the transparent bridging device as to which port this destination MAC Address can be reached via.

So instead of “flooding” the frame out of all ports; bare the one through which it arrived, the transparent bridging device will now be able to selectively and exclusively forward the frame out of only that port listed in the transparent bridging device’s Filter Table as being the port via which this destination MAC can be reached. This is what is known as filtering and it is also the mechanism used by transparent bridging devices to segment (break up) collision domains.

Filtering Benefits

Because only that segment on which the destination MAC Address resides receives the transmission all devices reachable via other ports of the transparent bridging device are free to engage in transmissions/conversations of their own. This results in considerable network performance improvements.

One Conversation/Segment – The only proviso here is that only one conversation per segment is permitted. In other words; multi-port transparent bridging devices enable a network to support multiple concurrent conversations but on a one conversation/segment basis.

Ethernet – Remember that transparent bridging devices are Ethernet devices and collisions are a fact of life for Ethernet networks. By segmenting collision domains transparent bridging devices improve network performance by increasing the number of concurrent transactions that the network supports while reducing the number of collisions that occur.

Traffic Localization – All conversations between nodes located on the same network segment will remain confined to that segment only. This allows other network segments to go about their business uninterrupted.

Plug ‘n’ Play – With plug ‘n’ play installation capabilities transparent bridging devices certainly reduce the amount of time spent installing/reinstalling devices. This allows for greater freedom and flexibility in network connectivity as well as reducing the amount of maintenance and administration required to keep the network up and running. Network reconfigurations are also accelerated when deploying/redeploying transparent bridging enabled devices.

Reduced User Interference – Reducing the amount of user input required produces additional benefits in that there are no user configurable parameters for “users” to wrongly configure or otherwise interfere with. In fact; most users will remain blissfully unaware of the transparent bridging device’s presence.


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