Configuring The Root Bridge Id - Cisco Nexus 6000 Series Configuration Manual

Configuring the Root Bridge ID

This example shows how to enable STP on a VLAN:
switch# configure terminal
switch(config)# spanning-tree vlan 5
Configuring the Root Bridge ID
The software maintains a separate instance of STP for each active VLAN in Rapid PVST+. For each VLAN,
the switch with the lowest bridge ID becomes the root bridge for that VLAN.
To configure a VLAN instance to become the root bridge, modify the bridge priority from the default value
(32768) to a significantly lower value.
When you enter the spanning-tree vlan vlan_ID root command, the switch checks the bridge priority of
the current root bridges for each VLAN. The switch sets the bridge priority for the specified VLANs to 24576
if this value will cause the switch to become the root for the specified VLANs. If any root bridge for the
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.
Cisco Nexus 6000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
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Command or Action
Caution
Purpose
Do not disable spanning tree on a VLAN unless all switches
and bridges in the VLAN have spanning tree disabled. You
cannot disable spanning tree on some of the switches and
bridges in a VLAN and leave it enabled on other switches
and bridges. This action can have unexpected results
because switches and bridges with spanning tree enabled
will have incomplete information regarding the physical
topology of the network.
Do not disable spanning tree in a VLAN without ensuring
that there are no physical loops present in the VLAN.
Spanning tree serves as a safeguard against
misconfigurations and cabling errors.
Configuring Rapid PVST+