Cisco Catalyst 2960 Series, Configuration Manual

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Path Cost Value Speed
2,000,000 10 Mb/s
200,000 100 Mb/s
20,000 1 Gb/s
2,000 10 Gb/s
200 100 Gb/s
Related Topics
Specifying the MST Region Configuration and Enabling MSTP , on page 261
Prerequisites for MSTP, on page 241
Restrictions for MSTP, on page 242
Spanning-Tree Interoperability and Backward Compatibility, on page 222
Optional Spanning-Tree Configuration Guidelines
BackboneFast, on page 294
UplinkFast, on page 290
Root Switch
The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A switch ID, consisting
of the switch priority and the switch MAC address, is associated with each instance. For a group of VLANs,
the switch with the lowest switch ID becomes the root switch.
When you configure a switch as the root, you modify the switch priority from the default value (32768) to a
significantly lower value so that the switch becomes the root switch for the specified spanning-tree instance.
When you enter this command, the switch checks the switch priorities of the root switches. Because of the
extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value
will cause this switches to become the root for the specified spanning-tree instance.
If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own
priority to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit
switch priority value. For more information, select "Bridge ID, Switch Priority, and Extended System ID"
link in Related Topics.
If your network consists of switches that support and do not support the extended system ID, it is unlikely
that the switch with the extended system ID support will become the root switch. The extended system ID
increases the switch priority value every time the VLAN number is greater than the priority of the connected
switches running older software.
The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure
an access switch as the spanning-tree primary root.
Use the
diameter
keyword, which is available only for MST instance 0, to specify the Layer 2 network
diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network).
When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
244
Information About MSTP