Defining the Spanning Tree Protocol – STP
Transparent bridges

Because switches have replaced bridges as the network device for implementing transparent bridging in modern networks, the basic functionality of a switch is identical to that of a transparent bridge on a per VLAN basis. To understand STP it is important first to look at the behavior of a transparent bridge without spanning tree.

By definition, a transparent bridge has these characteristics:

  • It must not modify the frames that are forwarded.
  • It learns addresses by "listening" on a port for the source address of a device. If a source MAC address is read in frames coming in a specific port, the bridge assumes that frames destined for that MAC address can be sent out of that port. The bridge then builds a table that records what source addresses are seen on what port. A bridge is always listening and learning MAC addresses in this manner.
  • It must forward all broadcasts out all ports, except for the port that initially received the broadcast.
  • If a destination address is unknown to the bridge, it forwards the frame out all ports except for the port that initially received the frame. This is known as unicast flooding.

Transparent bridging, by definition, must be transparent to the devices on the network. End stations require no configuration. The existence of the bridging protocol operation is not directly visible to them, hence the term transparent bridging.