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| 3.8 |  | RSTP and MST Configuration |
| 3.8.4 |  | Configuring the MST root switch |

The switch maintains a spanning tree instance for the group of VLANs mapped to it. A bridge ID, consisting of the switch priority and the switch MAC address, is associated with each instance. The switch with the lowest bridge ID becomes the root switch for the group of VLANs.

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 this command is issued, 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.

Catalyst 2950 switches running software earlier than Release 12.1(9)EA1 do not support the extended system ID. Catalyst 2950 switches running software earlier than Release 12.1(9)EA1 do not support the MSTP.

If the 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 specifying the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. Use the hello keyword to override the automatically calculated hello time.

It is recommended to avoid manually configuring the hello time, forward-delay time, and maximum-age time by using the spanning-tree mst hello-time, spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration commands after configuring the switch as the root switch.

Beginning in privileged EXEC mode, follow steps outlined in to configure a switch as the root switch.

To return the switch to its default setting, use the no spanning-tree mst instance-id root global configuration command.