Cisco Systems N3KC3048TP1GE, N3KC3064TFAL3 manual Configuring the Root Bridge ID

Configuring Rapid PVST+

Configuring the Root Bridge ID

Command or Action	Purpose
	Caution	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.

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.

Note The spanning-tree vlan vlan_ID root command fails if the value required to be the root bridge is less than 1.

Caution The root bridge for each instance of STP should be a backbone or distribution switch. Do not configure 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-timekeyword to override the automatically calculated hello time.

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
80	OL-26590-01