Huawei Quidway S3000 Series Operation Manual page 8

Operation Manual - STP
Quidway S3000 Series Ethernet Switches
Thus the spanning tree is stabilized. The tree with the root Switch A is illustrated in the
figure1-3 below.
Switch B
with priority 1
Figure1-3 The final stabilized spanning tree
To facilitate the descriptions, the description of the example is simplified. For example,
the root ID and the designated switch ID in actual calculation should comprise both
switch priority and switch MAC address. Designated port ID should comprise port
priority and port MAC address. In the updating process of a configuration BPDU, other
configuration BPDUs besides the first four items will make modifications according to
certain rules. The basic calculation process is described below:
Configuration BPDU forwarding mechanism in STP:
Upon the initiation of the network, all the switches regard themselves as the roots. The
designated ports send the configuration BPDUs of local ports at a regular interval of
HelloTime. If it is the root port that receives the configuration BPDU, the switch will
enable a timer to time the configuration BPDU as well as increase MessageAge carried
in the configuration BPDU by certain rules. If a path goes wrong, the root port on this
path will not receive configuration BPDUs any more and the old configuration BPDUs
will be discarded due to timeout. Hence, recalculation of the spanning tree will be
initiated to generate a new path to replace the failed one and thus restore the network
connectivity.
However, the new configuration BPDU as now recalculated will not be propagated
throughout the network right away, so the old root ports and designated ports that have
not detected the topology change will still forward the data through the old path. If the
new root port and designated port begin to forward data immediately after they are
elected, an occasional loop may still occur. In RSTP, a transitional state mechanism is
thus adopted to ensure the new configuration BPDU has been propagated throughout
the network before the root port and designated port begin to send data again. That is,
the root port and designated port should undergo a transitional state for a period of
Forward Delay before they enter the forwarding state.
Switch A
with priority 0
AP1
5
BP1
BP2
4
CP2
Switch C
with priority 2
1-6
Chapter 1 RSTP Configuration