C
HAPTER
8
| Spanning Tree Algorithm
Configuring Global Settings for STA
– 244 –
Advanced Configuration Settings
The following attributes are based on RSTP, but also apply to STP since the
switch uses a backwards-compatible subset of RSTP to implement STP, and
also apply to MSTP which is based on RSTP according to the standard:
◆ Path Cost Method – The path cost is used to determine the best path
between devices. The path cost method is used to determine the range
of values that can be assigned to each interface.
■
Long: Specifies 32-bit based values that range from 1-200,000,000.
(Thisisthedefault.)
■
Short: Specifies 16-bit based values that range from 1-65535.
◆ Transmission Limit – The maximum transmission rate for BPDUs is
specified by setting the minimum interval between the transmission of
consecutive protocol messages. (Range: 1-10; Default: 3)
When the Switch Becomes Root
◆ Hello Time – Interval (in seconds) at which the root device transmits a
configuration message.
■
Default: 2
■
Minimum: 1
■
Maximum: The lower of 10 or [(Max. Message Age / 2) -1]
◆ Maximum Age – The maximum time (in seconds) a device can wait
without receiving a configuration message before attempting to
reconverge. All device ports (except for designated ports) should
receive configuration messages at regular intervals. Any port that ages
out STA information (provided in the last configuration message)
becomes the designated port for the attached LAN. If it is a root port, a
new root port is selected from among the device ports attached to the
network. (References to “ports” in this section mean “interfaces,” which
includes both ports and trunks.)
■
Default: 20
■
Minimum: The higher of 6 or [2 x (Hello Time + 1)]
■
Maximum: The lower of 40 or [2 x (Forward Delay - 1)]
◆ Forward Delay – The maximum time (in seconds) this device will wait
before changing states (i.e., discarding to learning to forwarding). This
delay is required because every device must receive information about
topology changes before it starts to forward frames. In addition, each
port needs time to listen for conflicting information that would make it
return to a discarding state; otherwise, temporary data loops might
result.
■
Default: 15
■
Minimum: The higher of 4 or [(Max. Message Age / 2) + 1]
■
Maximum: 30
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