ATL Telecom R1-SW Ethernet Switch - Manual (Page 262)

R1-SW24L2B User’s Manual
13-4
When the switches in a network are powered up, each function operates as the root switch.
Each switch sends a configuration BPDU through all of its ports. The BPDUs communicate and
compute the spanning-tree topology. Each configuration BPDU contains this information:
y Unique bridge ID of the switch that the sending switch identifies as the root switch
y Spanning-tree path cost to the root
y Bridge ID of the sending switch
y Aging time of BPDU
y Interface ID that transmits BPDU
y Spanning tree timer values (Hello, Forward delay, Max-age)
Bridge ID determines the selection of the root switch. Each VLAN on the switch has a unique 8-
byte bridge ID; the two most-significant bytes are used for the switch priority, and the
remaining six bytes are derived from the switch MAC address. The switch with the highest
switch priority (the lowest numerical priority value) is elected as the root switch. If all switches
are configured with the default priority (32768), the switch with the lowest MAC address in the
VLAN becomes the root switch.
Path cost determines the selection of the root port and designated switch. The port that provides
the best path (lowest cost) when the switch forwards packets to the root switch is called the root
port. The switch that provides the lowest path cost when forwarding packets from that LAN to
the root switch is called the designated switch. The port through which the designated switch is
attached to the LAN is called the designated port.
BPDU has three spanning-tree timers (hello, forward delay, max age). The following table
describes the timers that affect the entire spanning-tree performance:
Table 13-1 STP Timers
Timer Description
Hello timer
When this timer expires, the interface sends out a Hello message to the neighboring
nodes.
Forward delay
timer Determines how long each of the listening and learning states last before the
interface begins forwarding.
Max age timer
Determines the amount of time the switch stores protocol information received on an
interface.