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Chapter 5

Using Spanning Tree with VLANs

a convergence problem that can be addressed when instability is occurring in
the network. The following timers can be changed:

fwddlay This interval indicates how long it takes for a port to move
from listening to learning state and then from learning to forwarding
state. The default is 15 seconds, but it can be changed to anywhere from
4 to 30 seconds. If you set this too low, the switch won't be allowed ample
time to make sure no loops will occur before setting a port in forwarding
mode. The following switch output shows how to set the fwddelay to 10
seconds:

Todd5000> (enable) set spantree fwddelay ?

Usage: set spantree fwddelay <delay> [vlans]

(delay = 4..30 seconds, vlan = 1..1005)

Todd5000> (enable) set spantree fwddelay 10

Spantree 1 forward delay set to 10 seconds.

hello This is the time interval for sending BPDUs from the root switch.
It is set to 2 seconds by default; you would think it couldn't be set any
lower, but it can. You can set it to 1 second to actually double the amount
of BPDUs sent out that must be lost before triggering an unwanted con-
vergence in the network. However, it doubles the CPU load and process-
ing load as well. The following switch output shows how to change the
BPDU timers to 1 second:

Todd5000> (enable) set spantree hello ?

Usage: set spantree hello <interval> [vlans]

(interval = 1..10, vlan = 1..1005)

Todd5000> (enable) set spantree hello 1

Spantree 1 hello time set to 1 seconds.

maxage The maxage is the amount of time that a switch will hold BPDU
information. If a new BPDU is not received before the maxage expires,
then the BPDU is discarded and is considered invalid. The default is 20
seconds; it can be set to as low as 6 seconds. However, network instability
will happen if too many BPDUs are discarded because this timer is set too
low. The following output shows how to change the maxage of a BPDU
to 30 seconds:

Todd5000> (enable) set spantree maxage ?

Usage: set spantree maxage <agingtime> [vlans]

(agingtime = 6..40, vlan = 1..1005)

Todd5000> (enable) set spantree maxage 30

Spantree 1 max aging time set to 30 seconds.

Todd5000> (enable)

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