HP 1910 User Manual page 205

Root bridge ID—Consisting of the priority and MAC address of the root bridge.
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• Root path cost—Cost of the path to the root bridge.
Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.
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Designated port ID—Designated port priority plus port name.
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• Message age—Age of the configuration BPDU while it propagates in the network.
Max age—Maximum age of the configuration BPDU can be maintained on a device.
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Hello time—Configuration BPDU interval.
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Forward delay—Delay used by STP bridges to transit the state of the root and designated ports to
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forwarding.
The descriptions and examples in this document involve only the following fields in the configuration
BPDUs:
Root bridge ID (represented by device priority).
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Root path cost.
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• Designated bridge ID (represented by device priority).
Designated port ID (represented by port name).
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Calculation process of the STP algorithm
The spanning tree calculation process described in the following sections is a simplified process for
example only.
The STP algorithm uses the following calculation process:
1. State initialization.
Upon initialization of a device, each port generates a BPDU with the port as the designated port,
the device as the root bridge, 0 as the root path cost, and the device ID as the designated bridge
ID.
2. Root bridge selection.
Initially, each STP-enabled device on the network assumes itself to be the root bridge, with its own
device ID as the root bridge ID. By exchanging configuration BPDUs, the devices compare their
root bridge IDs to elect the device with the smallest root bridge ID as the root bridge.
3. Root port and designated ports selection on the non-root bridges.
Table 54 Selecting the root port and designated ports
Step
1
2
Description
A non-root-bridge device regards the port on which it received the optimum
configuration BPDU as the root port.
BPDU is selected.
Based on the configuration BPDU and the path cost of the root port, the device
calculates a designated port configuration BPDU for each of the other ports.
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The root bridge ID is replaced with that of the configuration BPDU of the root port.
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The root path cost is replaced with that of the configuration BPDU of the root port plus
the path cost of the root port.
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The designated bridge ID is replaced with the ID of this device.
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The designated port ID is replaced with the ID of this port.
Table 55 describes how the optimum configuration
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