How Stp Works - HP A6600 Configuration Manual

How STP works

The devices on a network exchange BPDUs to identify the network topology. Configuration BPDUs
contain sufficient information for the network devices to complete spanning tree calculation. Important
fields in a configuration BPDU include:
• Root bridge ID: consisting of the priority and MAC address of the root bridge.
• Root path cost: the cost of the path to the root bridge denoted by the root identifier from the
transmitting bridge.
Designated bridge ID: consisting of the priority and MAC address of the designated bridge.
•
Designated port ID: designated port priority plus port name.
•
Message age: age of the configuration BPDU while it propagates in the network.
•
Max age: maximum age of the configuration BPDU.
•
Hello time: configuration BPDU transmission interval.
•
Forward delay: the delay used by STP bridges to transit the state of the root and designated ports to
•
forwarding.
Calculation process of the STP algorithm
Initial state
•
Upon initialization of a device, each port generates a BPDU with itself as the root bridge, in which the
root path cost is 0, designated bridge ID is the device ID, and the designated port is the port itself.
• Selection of the optimum configuration BPDU
Each device sends out its configuration BPDU and receives configuration BPDUs from other devices.
Table 8 describes the process of selecting the optimum configuration BPDU.
Table 8 Selection of the optimum configuration BPDU
Step
1
2
The following are the principles of configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
If all configuration BPDUs have the same root bridge ID, their root path costs are compared. Assume that
the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The configuration
BPDU with the smallest S value has the highest priority.
Actions
Upon receiving a configuration BPDU on a port, the device performs the following:
•
If the received configuration BPDU has a lower priority than that of the configuration
BPDU generated by the port, the device discards the received configuration BPDU and
does not process the configuration BPDU of this port.
•
If the received configuration BPDU has a higher priority than that of the configuration
BPDU generated by the port, the device replaces the content of the configuration BPDU
generated by the port with the content of the received configuration BPDU.
The device compares the configuration BPDUs of all ports and chooses the optimum
configuration BPDU.
43