From: Chris Lewis \(chrlewis\) (chrlewis@cisco.com)
Date: Mon Jun 20 2005 - 13:03:14 GMT-3
Hi,
I think QoS features are some of the worst documented of all, I know
this is a tough category to get prizes for, but anyway, here is how I
approach this sort of thing.
Router(config)#class-map test
Router(config-cmap)#match protocol ip
Router(config-cmap)#exit
Router(config)#policy-map test-PM
Router(config-pmap)#class test
Router(config-pmap-c)#police ?
<8000-2000000000> Bits per second
cir Committed information rate
rate Specify police rate
Router(config-pmap-c)#police 10000000 ?
<1000-512000000> Burst bytes
bc Conform burst
conform-action action when rate is less than conform burst
pir Peak Information Rate
<cr>
Router(config-pmap-c)#police 10000000 bc ?
<1000-512000000> Burst bytes
conform-action action when rate is less than conform burst
pir Peak Information Rate
<cr>
This shows the origin of the factor of 8 in your references, the rate
you police to has to be entered in bits per second, the burst value is
in bytes, a factor of 8 different. I have the feeling that the
specification of rate in bits and burst in bytes may have changed
between legacy and MQC syntax at some point, so that may be the origin
of confusion about this parameter.
The origin of the factor of 1.5 is somewhat different. This actually
depends on traffic profile and behavior to policing effects and there is
no hard and fast rule for what this factor should be. In general, if the
traffic is very bursty, a higher factor is warranted, if it is more well
order, a lower factor might be appropriate. This is assuming your goal
is to achieve an actual rate close to the configured policed rate.
This link is a fair explanation
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
fqos_c/fqcprt4/qcfpolsh.htm
This is one relevant extract.
"Testing of TCP traffic suggests that the chosen normal and extended
burst values should be on the order of several seconds worth of traffic
at the configured average rate. That is, if the average rate is 10 Mbps,
then a normal burst size of 10 to 20 Mbps and an Excess Burst size of 20
to 40 Mbps would be appropriate.
Recommended Burst Values
Cisco recommends the following values for the normal and extended burst
parameters:
normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds
extended burst = 2 * normal burst
With the listed choices for parameters, extensive test results have
shown CAR to achieve the configured rate. If the burst values are too
low, then the achieved rate is often much lower than the configured
rate"
For the purposes of the lab exam, I would suggest reading the question
extremely closely to determine what they are looking for in terms of
burst capability, given that there are no hard and fast rules about what
it should be.
The final point to be clear on here is the Bc is I think mistakenly
named a burst parameter, it is not really a burst above the policed
rate, it is the amount of data you can send out each Tc to reach CIR, Be
is the extended burst that relates to sending at rates above the policed
rate.
Chris
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Sheikh Rahman
Sent: Monday, June 20, 2005 9:18 AM
To: ccielab@groupstudy.com
Subject: burst vlaue
Hi
I am really confused with the burst vlaue with police command
In cco some document suggest burst vlaue is /8 to convert to byte then
*1.5
In other document or in the cisco press 642-642 book burst vlaue is /8.
Not multiply by 1.5
Could any one please guide me towards right direction?
Thanks
haf
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