Cisco Nexus 3000 Series Nx-Os Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1 - Cisco Nexus 3000 series Configuration Manual

Configuring the Root Bridge ID
specified VLANs has a bridge priority lower than 24576, the switch sets the bridge priority for the specified
VLANs to 4096 less than the lowest bridge priority.
The spanning-tree vlan vlan_ID root command fails if the value required to be the root bridge is less
Note
than 1.
The root bridge for each instance of STP should be a backbone or distribution switch. Do not configure
Caution
an access switch as the STP primary root.
Enter the diameter keyword to specify the network diameter (that is, the maximum number of bridge hops
between any two end stations in the network). When you specify the network diameter, the software
automatically selects an optimal hello time, forward delay time, and maximum age time for a network of that
diameter, which can significantly reduce the STP convergence time. You can enter the hello-time keyword
to override the automatically calculated hello time.
With the switch configured as the root bridge, do not manually configure the hello time, forward-delay
Note
time, and maximum-age time using the spanning-tree mst hello-time, spanning-tree mst forward-time,
and spanning-tree mst max-age configuration commands.
SUMMARY STEPS
1. switch# configure terminal
2. switch(config)# spanning-tree vlan vlan-range root primary [diameter dia [hello-time hello-time]]
DETAILED STEPS
Command or Action
Step 1 switch# configure terminal
Step 2 switch(config)# spanning-tree vlan vlan-range
root primary [diameter dia [hello-time
hello-time]]
This example shows how to configure the switch as the root bridge for a VLAN:
switch# configure terminal
switch(config)# spanning-tree vlan 5 root primary diameter 4

Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)

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Purpose
Enters configuration mode.
Configures a software switch as the primary root bridge. The
vlan-range value can be 2 through 4094 (except reserved VLAN
values.) The dia default is 7. The hello-time can be from 1 to 10
seconds, and the default value is 2 seconds.
Configuring Rapid PVST+