03-03-2010 08:48 AM - edited 03-04-2019 07:41 AM
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
03-03-2010 11:01 AM
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
05-05-2010 04:25 AM
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
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
05-05-2010 06:37 AM
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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