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Chapter 7
| Spanning Tree Algorithm
Configuring Global Settings for STA
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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
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Hello Time – Interval (in seconds) at which the root device transmits a
configuration message.
Default: 2
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Minimum: 1
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Maximum: The lower of 10 or [(Max. Message Age / 2) -1]
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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
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Minimum: The higher of 6 or [2 x (Hello Time + 1)]
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Maximum: The lower of 40 or [2 x (Forward Delay - 1)]
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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
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Minimum: The higher of 4 or [(Max. Message Age / 2) + 1]
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Maximum: 30
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RSTP does not depend on the forward delay timer in most cases. It is able to
confirm that a port can transition to the forwarding state without having to rely
on any timer configuration. To achieve fast convergence, RSTP relies on the use
of edge ports, and automatic detection of point-to-point link types, both of
which allow a port to directly transition to the forwarding state.
Configuration Settings for MSTP
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Max Instance Numbers – The maximum number of MSTP instances to which
this switch can be assigned.
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Configuration Digest – An MD5 signature key that contains the VLAN ID to
MST ID mapping table. In other words, this key is a mapping of all VLANs to the
CIST.
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