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Device
After the comparison processes described in 4, a spanning tree with Device A as the root bridge is
established, and the topology is shown in a.
The final calculated spanning tree
a.
NOTE:
This example shows a simplified spanning tree calculation process.
The configuration BPDU forwarding mechanism in STP
The configuration BPDUs of STP are forwarded following 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 it is the root port that 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 out 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 out its own configuration BPDU in response.
Comparison process
After comparison:
Because the root path cost of CP2 (9) (root path cost of the
BPDU (5) plus path cost corresponding to CP2 (4)) is smaller
than the root path cost of CP1 (10) (root path cost of the BPDU
(0) + path cost corresponding to CP2 (10)), the BPDU of CP2 is
elected as the optimum BPDU, and CP2 is elected as the root
port, the messages of which will not be changed.
After comparison between the configuration BPDU of CP1 and
the calculated designated port configuration BPDU, port CP1 is
blocked, with the configuration BPDU of the port unchanged,
and the port will not receive data from Device A until a
spanning tree calculation process is triggered by a new event,
for example, the link from Device B to Device C going down.
183
Configuration BPDU on
ports after comparison
Blocked port CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}

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