Cisco Catalyst 2950 Software Configuration Manual page 299

Chapter 14 Configuring MSTP
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
Catalyst 2950 switches running software earlier than Cisco IOS Release 12.1(9)EA1 do not support the
Note
extended system ID. Catalyst 2950 switches running software earlier than Cisco IOS Release
12.1(9)EA1 do not support the MSTP.
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,
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.
After configuring the switch as the root switch, we recommend that you avoid manually configuring the
Note
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
Step 1
configure terminal
Step 2 spanning-tree mst instance-id root primary
[diameter net-diameter [hello-time seconds]]
Step 3 end
Step 4
show spanning-tree mst instance-id
Step 5 copy running-config startup-config
78-11380-12
Table 13-1 on page 13-4.)
Purpose
Enter global configuration mode.
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 is 2 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.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
Configuring MSTP Features
14-15