Spanning Tree Algorithm Configuration

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3
Configuring Interface Settings
You can configure RSTP attributes for specific interfaces, including po rt priority, path
cost, link type, and edge port. You may use a different priority or path cost for ports
of the same media type to indicate the preferred path, link type to indicate a
point-to-point connection or shared-media connection, and edge port to indicate if
the attached device can support fast forwarding.
Command 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 on page 3-96 for additional information.)
-Discarding - Port receives STA configuration messages, but does no t 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:
•Spanning Tree – Enables/disables STA on this interface. (Default: Enabled).
•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 assigne d the
highest priority, the port with lowest numeric identifier will be enabled.
• Default: 128
• Range: 0-240, in steps of 16
•Path Cost – This parameter is used by the STP to determine the best path
between devices. Therefore, lower values should be assig ned to ports attached to
faster media, and higher values assigned to ports with slower media. (Pat h cost
takes precedence over port priority.) Note that when the Path Cost Meth od is set
to short (page 3-63), the maximum path cost is 65,535.
• Range –
- Ethernet: 200,000 -20,000,000
- Fast Ethernet: 20,000-2,000,000
- Gigabit Ethernet : 2,000-200,000
• Default –
- Ethernet – Half du plex: 2,000,000; full duplex: 1,000,000; trunk: 500,000
- Fast Ethernet – Half duplex: 200,000; full duplex: 100,000; trunk: 50,000
- Gigabit Ethernet – Full duplex: 10,000; trunk: 5,000