How Stp Works - HP 5820X series Configuration Manual

Path cost
Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects relatively
robust links and blocks redundant links, and finally prunes the network into a loop-free tree.

How STP works

STP has the following workflow:
Initial state
1.
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 root bridge
2.
Initially, each STP 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.
Selection of the root port and designated ports
3.
Table 10 describes the process of selecting the root port and designated ports.
Table 10 Selection of the root port and designated ports
Step
1
2
3
When the network topology is stable, only the root port and designated ports forward traffic. Other ports
are all in the blocked state in which the port receive BPDUs, but do not forward BPDUs or user traffic.
Description
A non-root device regards the port on which it received the optimum configuration BPDU
as the root port. For the selection of the optimum configuration BPDUs, see
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 rest ports.
• The root bridge ID is replaced with that of the configuration BPDU of the root port.
• The root path cost is replaced with that of the configuration BPDU of the root port plus
the path cost of the root port.
• The designated bridge ID is replaced with the ID of this device.
•
The designated port ID is replaced with the ID of this port.
The device compares the calculated configuration BPDU with the configuration BPDU on
the port whose port role is to be defined, and acts depending on the comparison result:
• If the calculated configuration BPDU is superior, the device considers this port as the
designated port, replaces the configuration BPDU on the port with the calculated
configuration BPDU, and periodically sends out the calculated configuration BPDU.
• If the configuration BPDU on the port is superior, the device blocks this port without
updating its configuration BPDU. The blocked port can receive BPDUs but not send
BPDUs or forward data traffic.
59
Table 11.