Digitial Equipment Corporation created Ethernet, and created transparent bridging for linking Ethernet segments. Since then, transparent bridging has been adapted for nearly every existing LAN protocol. A transparent bridge does not modify the frames it forwards in any way. To any device on the LAN, a transparent bridge is exactly that, transparent. End stations should be totally unaware that a bridge interconnects the two LAN segments, and should be invisible to the operation of the LAN.

Transparent bridges perform no protocol translation (such as from Ethernet to FDDI, or to Token Ring), and they do not calculate paths through the network to any end node. Bridges make their forwarding decisions based on the data link layer's MAC address.

  • Learning
  • Forwarding
  • Flooding
  • Filtering
  • Avoiding Loops

LEARNING

Transparent bridges actively listen to traffic on each segment on which it is attached. This is why transparent bridges are often called 'learning bridges'. A transparent bridge is listening to see where each MAC has been connected to the network. LAN segments are plugged into individual ports on the bridge. This enables the bridge to listen to each segment's traffic, and associate all the MAC's it 'hears' on that segment with that segment's port. This set of associations it keeps in a list in a special memory area called the 'cache'.

The device that has the destination MAC will respond to this frame, and since that destination machine is on only one segment, the response will come in on a specific port. The Bridge then knows which port the MAC is on. All future communication to that MAC will only be transmitted on that port. A bridge will also broadcast two other 'special' frame types: Multicast frames, and broadcast frames.

FORWARDING

When a transparent bridge encounters a frame that is to be forwarded to a destination MAC it forwards it out a specific port that it has associated with that MAC address.

FLOODING

If a bridge does not 'know' that MAC address (has no port associated with that MAC), it sends the frame out all the other ports on the bridge. In the figure below, a frame that has an unknown destination MAC comes in on the 'green' segment. The frame is 'flooded' out all the other ports on the segmengs drawn in red. The frame is NOT forwarded back out the green segment.

Flooding is also performed when a multicast or broadcast frame is received.

FILTERING

Frames are never forwarded out the port they are received on. When a bridge learns which segment a particular MAC address is on, it will ignore any frames transmitted by hosts ON that segment to the destination MAC that is on the same segment. All communication on the same segment between end nodes is ignored.


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