From: Edwards, Andrew M (andrew.m.edwards@boeing.com)
Date: Fri Dec 02 2005 - 19:33:07 GMT-3
Interesting... tha'ts not what I'm seeing.
I'm looking at this:
int f0/0
dampen 15
shut
As soon as I shut the interface down the penalty goes to 1000. Then
begins to decay at a half life of 15 seconds. The IOS may seem that its
decaying faster, but its doing it correctly. I say the IOS looks faster
because the decay is actually an exponentially decreasing logarithmic
decay function based on a half life of 15 seconds. So you should see
the penalty begin to decrement immediately according to the exponential
decay function.
That's all fine and dandy, but I think test wise, its important to just
reason out what is being asked of you.
For example, if you are asked to dampen the interface when it flaps 3
times in 30 seconds, you should realize with a default half life of 5
seconds, you will never make it dampen becuase the penalty decays too
fast. I think a more appropriate answer would be "dampen 30". This way
the compound penalty will be some number >2000 within 30 seconds.
Here is a long winded explaination of how I know the compound penalty is
> 2000. First flap at t=0 (penalty is 1000). For argument sake, say
the interface flaps again at t+15seconds. Without doing the full math,
it stands to reason that the accumulated penalty is greater than 500
(because we haven't reached the half life of 30 seconds yet) but less
than 1000. So the new penalty is greater than 1500 but less than 2000.
And now lets go one step further. For argument sake, lets say that the
interface flaps one more time at say t+29 seconds. Again, we haven't
reached the original half life, nor have we reached the second penalty
half life. And for this reason, the first half life penalty cannot be
less than 500 (because this would be the half life) and neither can the
second half life penalty be less than 500.
So, I have 2 penalties that are greater than 500. The summation of
these two penalties will yield a penalty that is greater than 1000. And
with the final penalty on the third flap of 1000, you can see we will be
at a compound penalty somewhere greater than 2000.
Since 2000 is the penalty marker to dampen the interface, we were
successful at meeting the requirement of dampening the interface if it
flaps 3 times within 30 seconds.
I tried to find a good site that discussed exponential decay, but its
been a while since I looked at logarithmic functions so they made no
sense to me... but knowing the general rule of half life you can get a
range as I dicsussed above that should help guide the course.
HTH,
andy
-----Original Message-----
From: Anthony Sequeira [mailto:terry.francona@gmail.com]
Sent: Friday, December 02, 2005 1:59 PM
To: Edwards, Andrew M
Cc: my-ccie-test@libero.it; ccielab@groupstudy.com
Subject: Re: IP Event Dampening
Yes - I saw that the half life default is actually 5 seconds.
But I also noticed that the penalty did not appear to be 1000.
Also - I set the half life to 15 seconds - yet the penalty decay
had nothing to do with 15 seconds. It would decay much more rapidly than
that.
It almost looks like Cisco is treating an interface shutdown
differently than some other interface failure.
How in the world are we to know how Cisco grades this
feature???? I can only imagine the response I would get from the proctor
on this one...."Do what the question states please. Thanks."
On 12/2/05, Edwards, Andrew M <andrew.m.edwards@boeing.com>
wrote:
Anthony,
Hopefully you saw that the deafult is 5 seconds and not
15 seconds as
documented?
To verify, use the command:
Show interface dampening
Seen on 12.2(15)T and even on 12.3
HTH,
Andy
CCIE #15334
-----Original Message-----
From: Anthony Sequeira [mailto: terry.francona@gmail.com
<mailto:terry.francona@gmail.com> ]
Sent: Friday, December 02, 2005 12:27 PM
To: my-ccie-test@libero.it
Cc: ccielab@groupstudy.com
Subject: Re: IP Event Dampening
Sad news everyone - I just labbed this up as well using
12.2(15)T16
code.....NONE of it seems to work as documented.
Indeed the default values are not what is
documented....AND - it took me
like 25 flaps before I could get the interface to
dampen. The penalty
assigned did not appear to be anything close to 1000 -
and the penalty
would decay at a rate much faster than the configured or
default Half
Life.
I only pray that Cisco grades this feature based on the
documentation -
and not on how the feature actually behaves in
production.
These are the things that make this journey rather
frustrating and at
times
- in fact - bordering on ludicrous.
On 12/2/05, my-ccie-test@libero.it
<my-ccie-test@libero.it> wrote:
>
> Hi Anthony,
> I did a lab cheking for this feature and wath I saw is
the following:
>
> first of all doing the command show interface
dampening it shows the
> default value for half-life time is 5 seconds (not
15..)
>
> #sh interfaces dampening
>
> ATM0/0
> Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV
MaxSTm MaxP
> Restart
>
> 0 0 FALSE 0 5 1000 2000
20
> 16000 0
>
> #
>
> I did a test trying to dampen an interface after 3
flap in 30 seconds.
> the config that I see work was with an half-life of 30
seconds and a
> suppress-threshold of 2500.
>
> I think if you want to dampen an interface after a
certain number of
> flap in a determined time you have to set suppress
thresold and also
> half-life time. wath do you think about ?
>
> thanks for any comments
>
>
This archive was generated by hypermail 2.1.4 : Mon Jan 09 2006 - 07:07:50 GMT-3