Selecting the Root Port

Selecting the Root Port
After the root bridge selection process is complete, all switches must calculate their cumulative
costs to the root bridge. Each switch listens to BPDUs on all active ports, and if BPDUs advertising
cost from the root are received on more than one port, the switch knows it has a redundant
link to the root bridge. Eventually, one switch in the loop has to determine which port will
become the root port and which port will be put into blocking state.
Spanning Tree Operation
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To determine the port that will be used to communicate with the root bridge, the path cost
is determined. The path cost is an accumulated total cost based on the bandwidth of the links.
The IEEE 802.1D specification proved to be unworkable with the introduction of higher speed
Ethernets, and the values were recently revised to handle the new higher speed links. Both cost
values are shown in Table 15.2.