HP FlexNetwork NJ5000 User Manual page 165

Figure 159 The final calculated spanning tree
The configuration BPDU forwarding mechanism of STP
The configuration BPDUs of STP are forwarded according to these guidelines:
•
Upon network initiation, every device regards itself as the root bridge, generates configuration
BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.
•
If the root port received a configuration BPDU and the received configuration BPDU is superior
to the configuration BPDU of the port, the device increases the message age carried in the
configuration BPDU following a certain rule, and it starts a timer to time the configuration BPDU
while sending this configuration BPDU through the designated port.
•
If the configuration BPDU received on a designated port has a lower priority than the
configuration BPDU of the local port, the port immediately sends its own configuration BPDU in
response.
•
If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs
and the old configuration BPDUs will be discarded because of timeout. The device generates
configuration BPDUs with itself as the root and sends the BPDUs and TCN BPDUs. This
triggers a new spanning tree calculation process to establish a new path to restore the network
connectivity.
However, the newly calculated configuration BPDU cannot be propagated throughout the network
immediately, so the old root ports and designated ports that have not detected the topology change
continue forwarding data along the old path. If the new root ports and designated ports begin to
forward data as soon as they are elected, a temporary loop might occur.
STP timers
STP calculation involves the following timers:
•
Forward delay—The delay time for device state transition. A path failure can cause spanning
tree recalculation to adapt the spanning tree structure to the change. However, the resulting
new configuration BPDU cannot propagate throughout the network immediately. If the newly
elected root ports and designated ports start to forward data immediately, a temporary loop is
likely to occur.
For this reason, as a mechanism for state transition in STP, the newly elected root ports or
designated ports require twice the forward delay time before they transit to the forwarding state,
which makes sure the new configuration BPDU has propagated throughout the network.
•
Hello time—The time interval at which a device sends hello packets to the neighboring devices
to make sure the paths are fault-free.
•
Max age—A parameter used to determine whether a configuration BPDU held by the device
has expired. The device discards the BPDU if the max age is exceeded.
153