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.