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Version 1.0
Step
Content
− Root path cost is replaced with the sum of the root path cost of the root port BPDU and
the path cost corresponding the root port.
− The ID of the designated bridge is replaced with that of the switch itself.
− The ID of the designated port is replaced with the port ID itself.
The switch compares the calculated BPDU with the BPDU of the port whose role requires to be
determined, and deals with the port according to different comparison results.
− If the calculated BPDU takes the precedence over the BPDU of the port, the port is chosen
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as the designated port with its BPDU replaced with the calculated BPDU, and regularly
sends out the BPDU.
− If the BPDU of this port takes the precedence over the calculated BPDU, the BPDU of this
port is not changed and the port is blocked. The port only receives BPDUs but cannot
forward BPDU or other data.
Tip
In a stable topology, only the root ports and designated ports can forward data, and other ports are blocked.
The blocked ports can only receive BPDUs, but not forward data.
STP Timer
◼
Hello Time
It specifies the interval for the root bridge to send BPDU messages to other switches, used to
test if the links malfunction.
◼
Maximum Aging Time
It specifies the maximum duration during which if a switch does not receive a BPDU message
from the root bridge, it sends BPDU packets to all the other switches for recalculating the new
STP.
◼
Forwarding Delay
It specifies the delay time the port state migration takes after the network topology changes.
Link malfunction leads to STP recalculation in the network, in which case, the STP structure will
change accordingly. However, as the new BPDUs cannot be spread to the whole network
immediately, the temporal loops might occur if the new root ports and the designated ports
forward data at once. Therefore, STP adopts a state migration mechanism, that is, the new root
ports and designated ports begin to forward data after twice forwarding delay, which ensures
the new BPDUs have been spread to the whole network.
Chapter 9 Device management 36