HPE FlexNetwork MSR Series User Manual

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• Forward delay—Delay used by STP bridges to transit the state of the root and designated
ports to forwarding.
For simplicity, the descriptions and examples in this document involve only the following fields in the
configuration BPDUs:
• Root bridge ID (represented by device priority)
• Root path cost (related to the rate of the link connecting the port)
• Designated bridge ID (represented by device priority)
• Designated port ID (represented by port name)
Calculation process of the STP algorithm
1. State initialization.
When you initialize 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 local
port.
2. Optimum configuration BPDU selection.
Each device sends out its configuration BPDU and receives configuration BPDUs from other
devices.
Table 137 Optimum configuration BPDU selection
Step Actions
1
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.
2
The device compares the configuration BPDUs of all the ports and chooses the
optimum configuration BPDU.
Configuration BPDU comparison uses the following principles:
 The configuration BPDU that has the lowest root bridge ID has the highest priority.
 If all the configuration BPDUs have the same root bridge ID, their root path costs are
compared. For example, 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.
 If all configuration BPDUs have the same S value, their designated bridge IDs, designated
port IDs, and the IDs of the receiving ports are compared in sequence. The configuration
BPDU containing a smaller ID is selected.
3. Root bridge selection.
Initially, each STP-enabled device on the network assumes itself to be the root bridge, with the
root bridge ID being its own device 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.
4. Root port and designated ports selection on a non-root device.
Table 138 Root port and designated ports selection
Step Description
1
A non-root device regards the port on which it received the optimum configuration
BPDU as the root port.

HPE FlexNetwork MSR Series Specifications

General

Brand	HPE
Model	FlexNetwork MSR Series
Category	Network Router
Language	English