05-31-2010 06:26 AM - edited 03-04-2019 08:38 AM
Hi Pros,
I ve been reading CB Shaping and Policing and got confused on the token bucket size in case of CB Policing - Single Rate Two Color.
Single Rate Two Color policers uses Single Token bucket. What is the bucket size in this case. Is the size equals BC? Because by default BC defaults to 1/4th of CIR. If I configure CIR as 96kbps, BC is set to 3000 bytes. Actually it should be set to 12000 bytes.
I am able to get the bucket size in case of Single Rate Three Colour (uses two token buckets) and Two Rate Three Colour (uses two token buckets). In both the cases, the bucket sizes are Bc and Be correspondingly. (only diff is how the buckets are replenished).
Could anyone clarify me on the bucket size in case of Single Rate Two Colour CB Policer?
thanks in advance.
arun
06-04-2010 06:00 AM
Hello Arun,
there is only one bucket in single rate two colors policer and that bucket is expressed in byte with rate expressed in bps
so to accomodate 96 kbps you need 96000/8 = 12000 bytes as you noted if we think of a 1 second time interval, but in policing what counts is the interarrival time between two packets and their respective packet size.
it is not like with shaping where there is a reference time interval that is part of the equation.
if a packet arrives at time t1 with size s1 and it is conforming it will drain s1 tokens.
If a second packet arrives at time t2 with size2 it is conforming if there are at least s2 tokens in the bucket
So I can police to 96000 bps even with a bucket smaller then 12000 bytes what changes is that initial burst that can be served without dropping is smaller and equivalent to 250 msec of traffic at specified rate.
from 12.4T QoS command reference
Token Bucket Algorithm with One Token Bucket
The one-token bucket algorithm is used when the violate-action option is not specified in the police command CLI.
The conform bucket is initially set to the full size (the full size is the number of bytes specified as the normal burst size).
When a packet of a given size (for example, "B" bytes) arrives at specific time (time "T"), the following actions occur:
•Tokens are updated in the conform bucket. If the previous arrival of the packet was at T1 and the current time is T, the bucket is updated with (T - T1) worth of bits based on the token arrival rate. The token arrival rate is calculated as follows:
(time between packets (which is equal to T - T1) * policer rate)/8 bytes
•If the number of bytes in conform bucket B is greater than or equal to the packet size, the packet conforms and the conform action is taken on the packet. If the packet conforms, B bytes are removed from the conform bucket and the conform action is completed for the packet.
•If the number of bytes in conform bucket B (minus the packet size to be limited) is fewer than 0, the exceed action is taken.
see
http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_n1.html#wp1060117
Hope to help
Giuseppe
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