Configuring The Root Switch - Cisco Catalyst 2950 Software Configuration Manual

Configuring Spanning-Tree Features

Configuring the Root Switch

The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge
ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For
each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.
To configure a switch to become the root, use the spanning-tree vlan vlan-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 VLAN. When you enter this command, the
switch checks the switch priority of the root switches for each VLAN. Because of the extended system
ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will cause this
switch to become the root for the specified VLAN.
If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own
priority for the specified VLAN 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
Note The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the
root switch is less than 1.
Before 12.1(9)EA1, entering the spanning-tree vlan vlan-id root global configuration command on a
Catalyst 2950 switch (no extended system ID) caused it to set its own switch priority for the specified
VLAN to 8192 if this value caused this switch to become the root for the specified VLAN. If any root
switch for the specified VLAN has a switch priority lower than 8192, the switch sets its own priority for
the specified VLAN to 1 less than the lowest switch priority.
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the
extended system ID support:
•
•
Note 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
Note
configure an access switch as the spanning-tree primary root.
Use the diameter keyword 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.
Catalyst 2950 Desktop Switch Software Configuration Guide
10-12
For Catalyst 2950 switches with the extended system ID (Release 12.1(9)EA1 and later), if all
network devices in VLAN 20 have the default priority of 32768, entering the spanning-tree vlan
20 root primary command on the switch sets the switch priority to 24576, which causes this switch
to become the root switch for VLAN 20.
For Catalyst 2950 switches without the extended system ID (software earlier than Release
12.1(9)EA1), if all network devices in VLAN 100 have the default priority of 32768, entering the
spanning-tree vlan 100 root primary command on the switch sets the switch priority for
VLAN 100 to 8192, which causes this switch to become the root switch for VLAN 100.
Chapter 10 Configuring STP
Table 10-1 on page 10-4.)
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