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After the root bridge
has been elected, all switches determine the best loop-free path to
the root. STP uses several different costs in determining the best
path to the root bridge as follows:
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Path cost
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Bridge ID
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Port priority
As the BPDU leaves a
port, the BPDU applies a port cost. The sum of all of the
port costs is the path cost. Spanning Tree looks first at the
path cost to decide which ports should forward and which ports
should block. If the path cost is equal for multiple ports, then
Spanning Tree looks at the bridge ID. If the bridge IDs are equal,
then STP compares the port IDs. The port with the lower port ID has
priority, making that port the forwarding port. Cisco allows you to
modify the first byte of the port ID by changing the port
priority.
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