HP FlexNetwork NJ5000, User Manual

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Path cost
Path cost is a reference value used for link selection in STP. STP calculates path costs to select the
most robust links and block redundant links that are less robust, to prune the network into a loop-free
tree.
All the ports on the root bridge are designated ports.
Calculation process of the STP algorithm
The spanning tree calculation process described in the following sections is a simplified process for
example only.
Calculation process
The STP algorithm uses the following calculation process:
1.
Network 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.
Step Description
1
A non-root-bridge device regards the port on which it received the optimum
configuration BPDU as the root port.
Table 60 describes how the optimum
configuration BPDU is selected.
2
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.
•
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.
3
The device compares the calculated configuration BPDU with the configuration BPDU
on the port whose port role will be determined, and acts depending on the result of the
comparison:
•
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 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 it cannot send BPDUs or forward any data.
When the network topology is stable, only the root port and designated ports forward user traffic.
Other ports are all in the blocked state to receive BPDUs but not to forward BPDUs or user
traffic.