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D-Link DHS-3224V Switch User’s Guide
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The switch sends BPDUs to communicate and construct the spanning-tree topology. All switches connected to
the LAN receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the
information in the frame to calculate a BPDU, and, if the topology changes, initiates a BPDU transmission.
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
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. The goal is to make the fastest link the root port.
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.