HP 200 Series Services And Applications page 59

Next, end node K receives two copies of the frame originated by end node J.
While the reception of duplicate frames by a node is not generally fatal, at
best such duplication represents an inefficient use of available bandwidth.
Of graver consequence is the effect of duplicate frames on bridge 1 and
bridge 2. The frame flooded by bridge 1 onto interface 1 is ultimately read by
bridge 2 on interface 2. When bridge 2 reads this frame, it updates its
address table to indicate end node J is in the direction of the white LAN. In a
similar fashion, bridge 1 reads the frame flooded by bridge 2, and it updates
its address table to show end node J in the direction of the white LAN.
Consequently, the address tables of both bridges are corrupted and neither
bridge is now able to properly forward a frame to end node J.
This corruption is caused by the existence of alternate routes between hosts.
Such alternate routes are generally referred to as loops. The spanning tree
algorithm (fully described in IEEE 802.1 MAC Bridges) ensures the
existence of a loop-free topology in networks that contain parallel bridges.
The algorithm provides a single path (composed of bridges and intervening
LANs) between any two nodes in such an extended network. It also provides
a high degree of fault tolerance by allowing for the automatic reconfigura-
tion of the spanning tree topology in the face of bridge or data-path failure.
Five management-assigned values are required for derivation of the
spanning tree topology:
A multicast address specifying all bridges within the extended
network
A network-unique identifier for each bridge within the extended
network
A unique identifier for each bridge/LAN interface (called a port)
A priority specifying the relative priority of each port
A cost for each port
With these values assigned, bridges broadcast and process formatted frames
(called bridge protocol data units or BPDUs) to derive a single loop-free
topology throughout the extended network. BPDU frame exchange is
accomplished quickly, thus minimizing the time du which service is
unavailable between hosts.
In constructing a loop-free topology, the bridges within the extended
network first determine the root bridge, the bridge with the best (that is,
lowest) priority value. This bridge serves as the root of the loop-free
topology.
Bridging Service
Transparent Bridging
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