Huawei Quidway S3000 Series Operation Manual page 33

Operation Manual - STP
Quidway S3000 Series Ethernet Switches
CP2 will receive the updated configuration BPDU, {0, 5, 1, BP2}, from Switch B. Since
this configuration BPDU is better then the old one, the old BPDU will be updated to {0,
5, 1, BP2}.
Meanwhile, CP1 receives the configuration BPDU from Switch A but its configuration
BPDU will not be updated and retain {0, 0, 0, AP2}.
By comparison, the configuration BPDU of CP2 is elected as the optimum one, CP2 is
elected as the root port, whose BPDU will not change, while CP1 will be blocked and
retain its BPDU, but it will not receive the data forwarded from Switch A until spanning
tree calculation is triggered again by some changes. For example, the link from Switch
B to C as down.
Thus the spanning tree is stabilized. The tree with the root Switch A is illustrated in the
Figure2-5
Switch B
with priority 1
Figure2-5 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
below.
Switch A
with priority 0
AP1
5
BP1
BP2
4
Switch C
with priority 2
CP2
2-9
Chapter 1 RSTP Configuration