Spanning Tree Algorithm Configuration

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3

Configuring Interface Settings for MSTP

You can configure the STA interface settings for an MST Instance using the MSTP
Port Configuration and MSTP Trunk Configuration pages.
Field Attributes
The following attributes are read-only and cannot be changed:
•STA State – Displays current state of this port within the Spanning Tree. (See
"Displaying Interface Settings for STA" on page 3-151 for additional information.)
-Discarding – Port receives STA configuration messages, but does not forward
packets.
-Learning – Port has transmitted configuration messages for an interval set by
the Forward Delay parameter without receiving contradictory information. Port
address table is cleared, and the port begins learning addresses.
-Forwarding – Port forwards packets, and continues learning addresses.
•Trunk – Indicates if a port is a member of a trunk. (STA Port Configuration only)
The following interface attributes can be configured:
•MST Instance ID – Instance identifier to configure. (Default: 0)
•Priority – Defines the priority used for this port in the Spanning Tree Protocol. If
the path cost for all ports on a switch are the same, the port with the highest priority
(i.e., lowest value) will be configured as an active link in the Spanning Tree. This
makes a port with higher priority less likely to be blocked if the Spanning Tree
Protocol is detecting network loops. Where more than one port is assigned the
highest priority, the port with lowest numeric identifier will be enabled.
(Default: 128; Range: 0-240, in steps of 16)
•Admin MST Path Cost – This parameter is used by the MSTP to determine the
best path between devices. Therefore, lower values should be assigned to ports
attached to faster media, and higher values assigned to ports with slower media.
(Path cost takes precedence over port priority.) Note that when the Path Cost
Method is set to short (page 3-63), the maximum path cost is 65,535.
By default, the system automatically detects the speed and duplex mode used on
each port, and configures the path cost according to the values shown below. Path
cost “0” is used to indicate auto-configuration mode. When the short path cost
method is selected and the default path cost recommended by the IEEE 8021w
standard exceeds 65,535, the default is set to 65,535
The recommended range is listed in Table 3-12 on page 3-155.
The recommended path cost is listed in Table 3-13 on page 3-155.
The default path costs are listed in Table 3-14 on page 3-156.