From: simon hart (simon@harttel.com)
Date: Mon Oct 24 2005 - 18:15:25 GMT-3
Hi Milenko,
You bring up some fascinating points with regards to this suppression
question. However I believe the question related to interface dampening as
opposed to BGP dampening, thus the times are expressed in seconds rather
than minutes, thus the rate at which the penalty decreases is far more rapid
(however the overall philosophy still holds).
So if we take the worst case scenario, do we use a suppress of 3000 ?? If
so the interface would be dampened after 4 flaps and not 5. Anything used
as a suppress-threshold of less than 4000 will mean the interface could get
dampened after 4 flaps!!!, But also means that the interface could
potentially not get dampened after 30 seconds.
And you are right regarding the reuse threshold, in the example I gave there
is potential for the route to get unsurpressed within that 30 second period.
However the question does not state how long it should be surpressed for, so
becoming unsurpressed within 30 seconds should not be an issue.
My conclusion is that it is impossible to answer this question with values
that will meet every criteria for flapping. If I had this question in the
lab, I think I would argue the point with the proctor.
I think the question would only make sense if it was geared to 2 flaps in 30
seconds or 3 flaps in 30 seconds.
More thoughts ?????
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Milenko Markov
Sent: 24 October 2005 21:44
To: 'simon hart'
Cc: ccielab@groupstudy.com
Subject: RE: Route Dampening
Simon,
lets observe "worst case scenario" (?) : 4 flaps "in" first second, and 1
flap in last (30').
Penalty would be (4x1000/2)+1000=3000...route would not be suppressed ?
I *think* that here two more values playes their role (marked with *) :
1. "Half-life-Once the route has been assigned a penalty, the penalty is
decreased by half after the half-life period (which is 15 minutes by
default). The process of reducing the penalty happens every *5 seconds*."
and
2. " Reuse limit-As the penalty for a flapping route decreases and falls
below this reuse limit, the route is unsuppressed. That is, the route is
added back to the BGP table and once again used for forwarding. The default
reuse limit is 750. The process of unsuppressing routes occurs at
*10-second* increments. Every 10 seconds, the router finds out which routes
are now unsuppressed and advertises them to the world."
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr
_c/ipcprt2/1cfbgp.htm#wp1002395
more ideas ?
regards,
Milenko.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: 24. oktobar 2005 22:31
To: Charles Cahoon; ccielab@groupstudy.com
Subject: RE: Route Dampening
Charles,
You will get a penalty of 1000 for each flap. Your half-life will default
to 30 seconds (best to leave it there).
You need to work out what to suppress at
If you flapped immediately 5 times in the first couple of seconds you will
get a penalty of 5000, however a figure of 5000 does not work in the
following scenario:-
If you flapped 2 times in the first second = 2000 and then flapped 3 times
in the last couple of seconds you will get a penalty of
roughly 2000/2 = 1000 + 3000 Therefore you will need a suppress-threshold
of 4000 or above, but below 5000
I would probably go for 4001 - any comments on this figure appreciated.
So
Half-life = 30
Suppress-threshold = 4001
Reuse-threshold = Arbitary for this exercise, however I would go with Cisco
recommendation of suppress-threshold = reuse*2, therefore 2000
Max-suppress = leave at default (60)
Simon
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Charles Cahoon
Sent: 24 October 2005 20:30
To: ccielab@groupstudy.com
Subject: Route Dampening
Looking for a simple explanation,
I have read a great deal about how to dampen but it's not getting any
clearer
for me. I was hoping that someone can help.
My question is if I want to remove a route if it flaps 5 times in 30
seconds?
Using the "Half-Life, Suppress Threshold, Reuse Threshold and Maximum
Suppress
time" not sure these correlate, what am I missing?
Thanks for the help
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