HP A5120 EI Series Configuration Manual page 64

Step Description
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.

2 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 will be defined, and acts depending on the result of the
comparison.

If the calculated configuration BPDU is superior, the device considers this port as the
3 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 not send
BPDUs or forward data traffic.
NOTE:
When the network topology is stable, only the root port and designated ports forward traffic, and other
ports are all in the blocked state in which the port receive BPDUs but do not forward BPDUs or user
traffic.
Table 9 Selection of the optimum configuration BPDU
Step 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
1 keeps the configuration BPDU this port generated.

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 the ports and chooses the optimum
2
configuration BPDU.
NOTE:
The following are the principles of configuration BPDU comparison:

The configuration BPDU with 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. 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 root path cost, their designated bridge IDs, designated port IDs, and
the IDs of the receiving ports are compared in sequence. The configuration BPDU that contains the smallest ID
wins.
A tree topology forms upon successful election of the root bridge, the root port on each non-root bridge
and the designated ports.
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