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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
BPDU that may return the
port to the blocking state.
Table 5-1. STP Parameters – Switch Level
The following are the user-configurable STP parameters for the port or port group level:
Variable Description Default
Value
Port Priority A relative priority for each
port – lower numbers give a
higher priority and a greater
chance of a given port being
elected as the root port
128
Port Cost A value used by STP to
evaluate paths – STP
calculates path costs and
selects the path with the
minimum cost as the active
path.
19 – 100Mbps
Fast Ethernet
ports
10 –
1000Mbps
Gigabit
Ethernet
ports
Table 5-2. STP Parameters – Port Group Level
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, rather 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
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