HP 200 Series Services And Applications page 60

Bridging Service
Transparent Bridging
After determining the identity of the root bridge, all other bridges calculate
path costs, that is the cost of the path to the root bridge offered by each
bridge port. Each bridge designates the port that offers the lowest-cost path
to the root bridge as the root port. In the event of equal path costs, the
bridge designates the port with the best (that is, lowest) priority value as the
root port.
On each LAN within the extended network, one bridge (the one whose root
port offers the lowest-cost path to the root bridge) is selected as the desig-
nated bridge. The port that connects the LAN to the designated bridge is
selected as the designated port. This port—said to be in the forwarding
state—carries all extended network traffic to and from the LAN.
This process ensures that all redundant ports (those providing parallel
connections) are removed from service (placed in the blocking state). In the
event of a topological change, or in the event of bridge or data-path failure,
however, the algorithm derives a new spanning tree that may move some
such ports from the blocking to the forwarding state.
Using figure 1 as an example, the implementation of the spanning tree
algorithm could remove bridge 1 from service and block bridge 2/interface 3.
Figure 2, below, shows the resulting logical topology that provides a
loop-free topology with only a single path between any two end nodes.
3 4
2
Red LAN
Blue LAN
Black LAN
White LAN
Figure 2. Spanning Tree (Loop-Free) Logical Topology
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