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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.