User’s Manual of GS-5220 LCD Series
225
Ports included in the STP are selected.
Creating a Stable STP Topology
It is to make the root port a fastest link. If all switches have STP enabled with default settings, the switch with
the lowest MAC address in the network will become the root switch. By increasing the priority (lowering the
priority number) of the best switch, STP can be forced to select the best switch as the root switch.
When STP is enabled using the default parameters, the path between source and destination stations in a
switched network might not be ideal. For instance, connecting higher-speed links to a port that has a higher
number than the current root port can cause a root-port change.
STP Port States
The BPDUs take some time to pass through a network. This propagation delay can result in topology changes
where a port that transitioned directly from a Blocking state to a Forwarding state could create temporary data
loops. Ports must wait for new network topology information to propagate throughout the network before
starting to forward packets. They must also wait for the packet lifetime to expire for 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
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