Cisco Systems 3560 Manual Do Utilizador

To return to the default MST region configuration, use the no spanning-tree mst configuration global 
configuration command. To return to the default VLAN-to-instance map, use the no instance instance-id 
[vlan vlan-range] MST configuration command. To return to the default name, use the no name MST 
configuration command. To return to the default revision number, use the no revision MST configuration 
command. To re-enable PVST+, use the no spanning-tree mode or the spanning-tree mode pvst global 
configuration command. 

This example shows how to enter MST configuration mode, map VLANs 10 to 20 to MST instance 1, 
name the region region1, set the configuration revision to 1, display the pending configuration, apply the 
changes, and return to global configuration mode:

Switch(config)# spanning-tree mst configuration

Switch(config-mst)# instance 1 vlan 10-20

Switch(config-mst)# name region1

Switch(config-mst)# revision 1

Switch(config-mst)# show pending

Pending MST configuration

Name      [region1]

Revision  1

Instance  Vlans Mapped

--------  ---------------------

0         1-9,21-4094

1         10-20

-------------------------------

Switch(config-mst)# exit

Switch(config)#

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. 

To configure a switch to become the root, use the spanning-tree mst instance-id root global 
configuration command to 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 switch 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 as shown in 

If your network consists of switches that both do 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,