D-Link DES-3326 manual Bridge Protocol Data Units, Variable Description Default Value

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	128
	port – lower numbers give a
	higher priority and a greater
	chance of a given port being
	elected as the root port
Port Cost	A	value	used	by	STP to		19 – 100Mbps
	evaluate		paths		–	STP	Fast Ethernet
	calculates		path	costs		and	ports
	selects the path with the
	minimum cost as the active						10	–
	path.						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

31