Changing the STP Timers

Changing the STP Timers
The timers are important in an STP network to stop network loops from occurring. The different
timers are used to give the network time to update the correct topology information to all
the switches and also to determine the whereabouts of all the redundant links.
The problem with the STP timers is that, if a link goes down, it takes up to 50 seconds for
the backup link to take over forwarding frames. This is a convergence problem that can be
addressed when instability is occurring in the network. The following timers can be changed:
fwddelay 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:
Terry_4000> (enable) set spantree fwddelay ?
Usage: set spantree fwddelay <delay> [vlans]
(delay = 4..30 seconds, vlan = 1..1005)
Terry_4000> (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 be increased or decreased. You can
set it to 1 second to actually double the amount of BPDUs sent out that must be lost before triggering
an unwanted convergence in the network. However, it doubles the CPU load and processing load
as well. The following switch output shows how to change the BPDU timers to 1 second:
Terry_4000> (enable) set spantree hello ?
Usage: set spantree hello <interval> [vlans]
(interval = 1..10, vlan = 1..1005)
Terry_4000> (enable) set spantree hello 1
Spantree 1 hello time set to 1 seconds.
maxage The max age is the amount of time that a switch will hold BPDU information. If a new
BPDU is not received before the max age 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 max age of a BPDU to 30 seconds:
Terry_4000> (enable) set spantree maxage ?
Usage: set spantree maxage <agingtime> [vlans]

(agingtime = 6..40, vlan = 1..1005)
Terry_4000> (enable) set spantree maxage 30
Spantree 1 max aging time set to 30 seconds.
Terry_4000> (enable)
Rather than directly modifying the timers, it is usually better to modify the size of the network.
Table 16.1 referred to a “diameter” value that can be set when selecting the spanning tree
root. The diameter used is the width of the network from one side to the other. Three switches
daisy-chained together would have a diameter of 3, whereas three configured in a triangle
would have a diameter of 2.
The diameter automatically sets the timers to a value appropriate to the size of your network. Setting
the timers yourself to low values in a large network risks topological loops because the delay
might not be long enough to account for BPDU propagation delay. The best thing to do is to use the
diameter option when setting the root and then modify the timers from there, if necessary.
We have been discussing redundant links and STP, but most of the discussion has been about
how to make STP run efficiently, and that is by making the non-root port a blocking port. We
discussed load balancing only when we showed you how to set the port priority on a per-VLAN
basis. However, that really wasn’t load balancing to the degree that is possible with a Cisco
switched network. In the next section, we’ll cover the most efficient ways of using redundant
links in a large, switched internetwork.
To set similar parameters on the IOS-based switches, use the global command spanning-tree
vlan vlan_number options as follows:
Terry_2950(config)#spanning-tree ?
backbonefast Enable BackboneFast Feature
etherchannel Spanning tree etherchannel specific configuration
extend Spanning Tree 802.1t extensions
loopguard Spanning tree loopguard options
mode Spanning tree operating mode
pathcost Spanning tree pathcost options
pathcost Spanning tree pathcost options
uplinkfast Enable UplinkFast Feature
vlan VLAN Switch Spanning Tree
Terry_2950(config)#spanning-tree vlan 1 ?
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
priority Set the bridge priority for the spanning tree
root Configure switch as root
<cr>
Terry_2950(config)#spanning-tree vlan 1 forward-time ?
<4-30> number of seconds for the forward delay timer


Terry_2950(config)#spanning-tree vlan 1 hello-time ?
<1-10> number of seconds between generation of config BPDUs
Terry_2950(config)#spanning-tree vlan 1 max-age ?
<6-40> maximum number of seconds the information in a BPDU is valid