H3C Technologies H3C S12500-X Series Switches - Manual (Page 78)

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Figure 17 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 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 due to timeout. The device generates a configuration
BPDU 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
The most important timing parameters in STP calculation are forward delay, hello time, and max age.
• Forward delay
Forward delay is the delay time for port state transition.
A path failure can cause spanning tree re-calculation 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 will likely 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 to
make sure the new configuration BPDU has propagated throughout the network.
• Hello time
The device sends hello packets at the hello time interval to the neighboring devices to make sure
the paths are fault-free.
• Max age