IP event dampening

Unanswered Question
Mar 3rd, 2010
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G'Day everyone,

Wondering if anyone could help me out here. I trying

to get my head around this whole dampening thing. I know why you need it,

and how to configure it. The only thing I don't know or understand is how

to come up with the values (other then the default). Lets take ip event

dampening for an example Ok.


Router(config)#interface fa0/0

Router(config-if)#dampening [half-life-period reuse-threshold]

[suppress-threshold max-suppress] [restart-penalty]

Router(config-if)#end


Now I understand that once an interface flaps it is

assigned a penalty of 1000. Half-life period is the time after which the

penalty is decreased. The penalty is decreased by half after the half-life

period expires. The reuse-threshold is based on the number of penalise. Once

the penalty falls below this value the interface is unsuppressed. The

suppress-threshold is the value of the accumulated penalties. Once the

suppress-threshold is reached the interface(or route) is suppressed. The

max-suppress-time is basically the time in seconds the interface can be

suppressed (im a little confused with this compared to reuse-threshold). And

lastly the restart-penalty is just a value the interface should start with

once the router has been rebooted and the interface comes up.


Now with all this said, how does one work all these

values out. For example. I have an interface that flaps. I want to dampen

that interface if it flaps lets say 3 time in 5 minutes. once it is dampened

it should stay dampened for 10 mins. How would I work out these values for

this sort of problem. Is there some magic formula ? Its prob very simple but

Im just not seeing the light here. Any help would be appreciated.


Regards

Manoj
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Jerry Ye Wed, 03/03/2010 - 11:01
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  • Cisco Employee,

The max suppress time allowed to configure is 255 sec.


Half-file time for penalty (Range=1..30)
Re-use value (Range=1..20000)
Suppress Value (Range=1..20000)
Max suppress (Range=1..255)
Restart penalty (enabled/disable)
Penalty applied at re-start (Range=1..20000)


The Max penalty value can be calculated, and it is based on the BGP dampening formular


http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1039828


Max penalty = reuse-limit * 2^( max suppress time / half-time)


For example, if you have the interface dampening value as follow dampening 30 1 1 255


Max penalty = 1 x 2^(255/30)

Max penality = 2^(8.5)

                     = 362


Which match the router output


interface Serial0/0/0

...
  dampening 30 1 1 255

...

end


Serial0/0/0
  Flaps Penalty    Supp ReuseTm   HalfL  ReuseV   SuppV  MaxSTm    MaxP Restart
      0       0   FALSE       0      30       1       1     255     362       0


HTH,

jerry

manoj.3.joshi.con Wed, 05/05/2010 - 04:25
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Hi Jerry,

I agree with you on this. Routers with IOS version 12.4(13r)T are also providing 255 sec as upper limit.


But attached documet says Max. Suppress time can be configured from 1 to 20000 Seconds. The default is 20 seconds or four times of half cycle value.


http://www.cisco.com/en/US/docs/ios/12_2t/12_2t13/feature/guide/ftipevdp.html#wp1048368


quote

Maximum Suppress Time

The maximum suppress time represents the maximum amount of time an interface can remain dampened when a penalty is assigned to an interface. The maximum suppress time can be configured from 1 to 20000 seconds. The default of the maximum penalty timer is 20 seconds or four times the default half-life period (5 seconds). The maximum value of the accumulated penalty is calculated, based on the maximum suppress time, reuse threshold, and half-life period.


unquote

Jerry Ye Wed, 05/05/2010 - 06:37
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  • Cisco Employee,

I think there is a documentation mistake in the cisco.com links. I've checked the command output and the max configured value is 255


Rack1R1(config-if)#dampening 30 1 1 ?          
  <1-255>  Maximum duration to suppress an interface


HTH,

jerry

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