Specifications

Ethernet Bridge Operation 9-3
Spanning Tree Algorithm
Assume that station A wants to transmit to Station D. Station A sends the
frame to Station D. The bridge reads in the frame and checks the CRC. The
bridge reads the source address of the frame (Station A) and makes sure
Station A is still in the SAT. The bridge checks the SAT for the destination
address, Station D. It is not found to reside on the Network 1 side of the
bridge so the frame is forwarded to Network 2.
Next, Station D sends its response to Station A. The bridge reads in the
frame, and after checking the CRC, updates the SAT with the Station D
address as being located on Network 2. The SAT now shows the
following:
The bridge then inspects the SAT looking for Station A on the Network 2
side of the bridge. It was not found on the Network 2 side so the frame is
forwarded to Network 1.
This process continues until all stations are logged on the SAT. The bridge
will now isolate different network traffic, as well as extend the maximum
size of each individual network.
Spanning Tree Algorithm
Considering the important role bridges play in the transfer of data from
one network to another, it is a good idea to set up a redundant bridge that
commences operation if the primary bridge should fail. Therefore, IEEE
chose to build some fault tolerance into the bridge specification. The
802.1d specification defines bridge operation, redundancy and a process
called the Spanning Tree Algorithm (STA) which allows bridges to be
connected in such a way as to create standby redundant paths without
creating data loops. This same algorithm activates a redundant path in
case of a failure in the active path.
Network 1 Network 2
AD
B
It is important to realize that the bridge did not make the forwarding
decision because Station A was on the Network 1 side, but because
Station A was not on the Network 2 side.
NOTE