Configuring The Root Switch - Cisco NCS 4200 Series Configuration Manual
Lan switching
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Configuring PVST+ and RPVST+
BackboneFast
Configured Pathcost method used is short
Name
---------------------- -------- --------- -------- ---------- ----------
VLAN0001
VLAN0002
VLAN0003
VLAN0004
VLAN0005
VLAN0006
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 for the specified VLAN, 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. When you enter this command, the software 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 Table 14-1 on page 14-4 .)
The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the
Note
root switch is less than 1.
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.
Note
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 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.
Note
After configuring the switch as the root switch, we recommend that you avoid manually configuring the
hello time, forward-delay time, and maximum-age time through the spanning-tree vlanvlan-id hello-time,
spanning-tree vlanvlan-id forward-time, and the spanning-tree vlanvlan-id max-age global configuration
commands.
Beginning in privileged EXEC mode, follow these steps to configure a switch to become the root for the
specified VLAN. This procedure is optional.
is disabled
Blocking Listening Learning Forwarding STP Active
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LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)
Configuring the Root Switch
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