NS3503-16P-4C User Manual
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BPDU packets that were forwarded based on the old topology. The forward delay timer is used to allow
the network topology to stabilize after a topology change. In addition, STP specifies a series of states a
port must transition through to further ensure that a stable network topology is created after a topology
change.
Each port on a switch using STP exists is in one of the following five states:
Blocking – the port is blocked from forwarding or receiving packets
Listening – the port is waiting to receive BPDU packets that may tell the port to go back to the
blocking state
Learning – the port is adding addresses to its forwarding database, but not yet forwarding packets
Forwarding – the port is forwarding packets
Disabled – the port only responds to network management messages and must return to the
blocking state first
A port transitions from one state to another as follows:
From initialization (switch boot) to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
From learning to forwarding or to disabled
From forwarding to disabled
From disabled to blocking
Figure 4-6-1 STP Port State Transitions
You can modify each port state by using management software. When you enable STP, every port on
every switch in the network goes through the blocking state and then transitions through the states of
