Instructions

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Gigabit Ethernet PoE+ Web-Managed AV Switch User Manual
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.
STP PORT STATES
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 rst
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
It’s possible to 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
listening and learning at power up. If properly configured, each port stabilizes to the forwarding or blocking
state. No packets (except BPDUs) are forwarded from or received by STP enabled ports, until the forwarding
state is enabled for that port.