Bridging
Bridging is used to connect two network segments. This alleviates congestion problems on a single Ethernet
segment and extends allowed cabling distances because the segments on each side of the bridge conformed
to the same distance limitation as a single segment. This bridge is called "transparent bridging" because
the end-point devices do not need to know that the bridge exists.
Transparent bridges forward frames only when necessary and, thus, reduces network overhead. To
accomplish this, transparent bridges learning MAC addresses by examining the source MAC address of each
frame received by the bridge; decides when to forward a frame or when to filter a frame, based on the
destination MAC address; and creates a loop-free environment with other bridges by using the Spanning
Tree Protocol.
Generally, broadcasts and multicast frames are forwarded by the bridge in networks that use bridges. In
addition, transparent bridges perform switching of frames using Layer 2 headers and Layer 2 logic and are
Layer 3 protocol-independent. Store-and-forward operation, which means that the entire frame is received
before the first bit of the frame is forwarded, is also typical in transparent bridging devices. However, the
transparent bridge must perform processing on the frame, which also can increase latency.
A transparent bridge operates in the following manner:
• The bridge has no initial knowledge of the location of any end device; therefore, the bridge must listen
to frames coming into each of its ports to figure out on which network a device resides.
• The bridge constantly updates its bridging table upon detecting the presence of a new MAC address or
upon detecting a MAC address that has changed location from one bridge port to another. The bridge is
then able to forward frames by looking at the destination address, looking up the address in the bridge
table, and sending the frame out the port where the destination device is located.
the frame out all available ports. However, the frame is not forwarded out the port that initially received
the frame. Hence, broadcasts are able to reach all available networks. A bridge only segments collision
domains but does not segment broadcast domains.
• If a frame arrives with a destination address that is not found in the bridge table, the bridge is unable
to determine which port to forward the frame to for transmission. This is known as an unknown unicast.
In this case, the bridge treats the frame as if it was a broadcast and forwards it out all remaining ports.
After a reply to that frame is received, the bridge will learn the location of the unknown station and add it
to the bridge table.
• Frames that are forwarded across the bridge cannot be modified.