A secondary root is a switch that can become the root bridge for a VLAN if the primary
root bridge fails. To con?¬?gure a switch as the secondary root bridge for the VLAN, use the
command spanning-tree vlan vlan-ID root secondary.
With this command, the switch priority is modi?¬?ed from the default value of 32768 to
28672. Assuming that the other bridges in the VLAN retain their default STP priority, this
switch becomes the root bridge if the primary root bridge fails. You can execute this
command on more than one switch to con?¬?gure multiple backup root bridges.
Summary of Improving Performance with Spanning Tree
The following summarizes the key points that were discussed in this section.
?– A redundant switched topology includes redundantly connected switches and
EtherChannel.
CAUTION Spanning-tree commands take effect immediately, so network traf?¬?c is
interrupted while recon?¬?guration occurs.
64 Chapter 2: Medium-Sized Switched Network Construction
?– A redundant switched topology causes looping issues such as broadcast storms.
?– The 802.1D STP establishes a loop-free network.
?– The original STP has been enhanced by PVST+ and RSTP.
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