Interlogix NS3502-8P-2S User Manual - Manual (Page 143)

IFS NS3502-8P-2S User Manual
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 The path cost to the root from the transmitting port
 The port identifier of the transmitting port
The switch sends BPDUs to communicate and construct the spanning-tree topology. All switches
connected to the LAN on which the packet is transmitted will 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
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