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Configuring a Root Switch
Instances 2-4 bridge hello time set to 2 seconds.
Instances 2-4 bridge forward delay set to 9 seconds.
Switch is now the root switch for active Instances 1-6.
Console> (enable)
Configuring a Root Switch to Improve Convergence
You can configure the root switch to speed up STP convergence time. To do so, you must reduce the
value of the Hello Time, Forward Delay Timer, and Maximum Age Timer parameters. For information
on configuring these timers, see the
Reduction of the value of the timer parameters is possible only if all of the links are LAN links of
Note
10Mbps or faster. In this case, the network diameter can reach the maximum value of 7. With WAN
connections, it is not possible to reduce the parameters.
When a link failure occurs in a bridged network, network reconfiguration is not immediate.
Reconfiguration requires a time of 50 seconds, with the default parameters (specified by IEEE 802.1D)
for the Hello Time, Forward Delay Timer, and Maximum Age Timer. The reconfiguration delay depends
on the network diameter, which is the maximum number of bridges between any two points of
attachment of end stations.
To speed up convergence, we use nondefault parameters valuespermitted by the IEEE 802.1D standard.
Non-default parameters to set for a reconvergence of 14 seconds are as follows:
Parameter
Network Diameter (dia)
Hello Time
Forward Delay Timer
Maximum Age Timer
You can set these parameters on the Catalyst 4000 family switches without modifying the switches.
Switch ports can be set for improved convergence in PortFast mode. This setting affects only the
Note
transition from disable (link down) to enable (link up), moving the port immediately to the forwarding
state. If a port in PortFast mode begins blocking, then it goes through listening and learning before
reaching the forwarding state.
To configure the spanning tree bridge to improve convergence, perform this task in privileged mode:
Task
Step 1
Configure the Hello time for a VLAN or
MISTP instance.
Step 2
Verify the configuration.
Step 3
Configure the forward delay time for a
VLAN or MISTP instance.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
7-30
"Configuring Spanning Tree Timers" section on page
Time
2
2 seconds
4 seconds
6 seconds
Command
set spantree hello interval [vlan] mistp-instance
[instances]
show spantree [vlan | mistp-instance instances]
set spantree fwddelay delay [vlan] mistp-instance
[instances]
Chapter 7
Configuring Spanning Tree
7-30.
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