Instruction Manual
67
Bridge Protocol Data Units
For STP to arrive at a stable network topology, the following information is used:
•
The unique switch identifier.
•
The path cost to the root associated with each switch port.
•
The port identifier
STP communicates between switches on the network using Bridge Protocol Data
Units (BPDUs). Each BPDU contains the following information:
•
The unique identifier of the switch that the transmitting switch currently
believes is the root switch.
•
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
Using a stable STP topology makes the root port the 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.
