15-15
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
78-11380-10
Chapter15 Configuring MSTP Configuring MSTP Features
If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its
own priority to4096 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 Table14-1 on page 14-4.)
Note Catalyst 2950 switches running software earlier than Cisco IOS Release 12.1(9)EA1 do not support the
extended systemID. Catalyst 2950 switches running software earlier than Cisco IOS Release
12.1(9)EA1 do not support the MSTP.
Note If your network consists of switches that both do and do not support the extended sy stem 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.
Note The root switch for each spanning-tree instance shou ld b e a ba ckbon e or dist ribution sw it ch. D o no t
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 time, and maximum-age time for a network of that diameter, which can significantly
reduce the convergence time. You can use the hello keyword to override the automatically calculated
hello time.
Note After configuring the switch as the root switch, we recommend that you 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 these steps to configure a switch as the root switch. This
procedure is optional.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 spanning-tree mst instance-id root primary
[diameter net-diameter [hello-time seconds]] Configure a switch as the root switch.
•For instance-id, you can specify a single instance, a range
of instances separated by a hyphen, or a series of instances
separated by a comma. The range is 0 to 15.
•(Optional) For diameter net-diameter, specify the
maximum number of switches between any two end
stations. The range is2 to 7. This keyword is available
only for MST instance 0.
•(Optional) For hello-time seconds, specify the interval in
seconds between the generation of configuration messages
by the root switch. The range is 1 to 10 seconds; the
default is 2 seconds.