Chapter 7
| Spanning Tree Algorithm
Configuring Global Settings for STA
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◆ Cisco Prestandard Status – Configures spanning tree operation to be
compatible with Cisco prestandard versions. (Default: Disabled)
Cisco prestandard versions prior to Cisco IOS Release 12.2(25)SEC do not fully
follow the IEEE standard, causing some state machine procedures to function
incorrectly. The command forces the spanning tree protocol to function in a
manner compatible with Cisco prestandard versions.
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
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