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Bridge: Switch A is the root bridge in the whole network; switch B is the designated bridge of
switch C.
Port: Port 3 is the root port of switch B and port 5 is the root port of switch C; port 1 is the
designated port of switch A and port 4 is the designated port of switch B; port 6 is the
blocked port of switch C.
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.
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