Administrator Guide

Layer 2 Switching Commands 784
The default setting of 20 seconds is suitable for a network of diameter 7, lost
message value of 3, transit delay of 1, hello interval of 2 seconds, overestimate
per bridge of 1 second, and a BPDU delay of 1 second. For a network of
diameter 4, a setting of 16 seconds is appropriate if all other timers remain at
their default values. IEEE 802.1Q notes that RSTP and MSTP treat the
common spanning tree message age field as a hop count. Section 13.37
Performance discusses appropriate and recommended values and further
refers the network operator to the discussion in IEEE 802.1D section 17.14.
In particular, operators should make themselves of the requirement that
bridges must enforce the following constraint:
2 × (Bridge_Forward_Delay – 1.0 seconds) >= Bridge_Max_Age >= 2 ×
(Bridge_Hello_Time + 1.0 seconds)
Example
console(config)#spanning-tree vlan 3 max-age 18
spanning-tree vlan root
Use the spanning-tree vlan root primary command to configure the switch to
become the root bridge or standby root bridge by modifying the bridge
priority from the default value to a lower value calculated to ensure the bridge
is the root (or standby) bridge. Use the no form of the command to let the
network elect the root bridge.
Syntax
spanning-tree vlan vlan-list root {primary | secondary}
no spanning tree vlan vlan-list root
Default Configuration
The default bridge priority value is 32768.
Command Modes
Global Configuration mode