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The communication between switches via BPDUs results in the following:
。 One switch is elected as the root switch
。 The shortest distance to the root switch is calculated for each switch
。 A designated switch is selected. This is the switch closest to the root switch through which packets
will be forwarded to the root.
。 A port for each switch is selected. This is the port providing the best path from the switch to the root
switch.
。 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
A port transitions from one state to another as follows:
。 From initialization (switch boot) to blocking
。 From blocking to listening or to disabled