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Figure 7-1 Basic STP diagram
STP Timers
Hello Time:
Hello Time ranges from 1 to 10 seconds. It specifies the interval to send BPDU packets. It is used
to test the links.
Max. Age:
Max. Age ranges from 6 to 40 seconds. It specifies the maximum time the switch can wait without
receiving a BPDU before attempting to reconfigure.
Forward Delay:
Forward Delay ranges from 4 to 30 seconds. It specifies the time for the port to transit its state
after the network topology is changed.
When the STP regeneration caused by network malfunction occurs, the STP structure will get
some corresponding change. However, as the new configuration BPDUs cannot be spread in the
whole network at once, the temporal loop will occur if the port transits its state immediately.
Therefore, STP adopts a state transit mechanism, that is, the new root port and the designated
port begins to forward data after twice forward delay, which ensures the new configuration BPDUs
are spread in the whole network.
BPDU Comparing Principle in STP mode
Assuming two BPDUs: BPDU X and BPDU Y
If the root bridge ID of X is smaller than that of Y, X is superior to Y.
If the root bridge ID of X equals that of Y, but the root path cost of X is smaller than that of Y, X is
superior to Y.
If the root bridge ID and the root path cost of X equal those of Y, but the bridge ID of X is smaller
than that of Y, X is superior to Y.
If the root bridge ID, the root path cost and bridge ID of X equal those of Y, but the port ID of X is
smaller than that of Y, X is superior to Y.
STP Generation
In the beginning
In the beginning, each switch regards itself as the root, and generates a configuration BPDU for
each port on it as a root, with the root path cost being 0, the ID of the designated bridge being that
of the switch, and the designated port being itself.
Comparing BPDUs
Each switch sends out configuration BPDUs and receives a configuration BPDU on one of its ports
from another switch. The following table shows the comparing operations.
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