12-10-2012 08:31 AM - edited 03-04-2019 06:22 PM
Hi
I want to configure a qos policy for a dmvpn network.
The ideal is to set a pir of 2048000, with cir 512000.
I believe shape peak will do the job, but I'm completely lost on the calculation.
Can anyone help me please with the math.
Thanks a lot!
Phil
Solved! Go to Solution.
12-10-2012 06:31 PM
Hi Philip,
Yes this is perfect.
Formula no.1 : Tc = Bc/CIR
10ms = Bc/512000
Bc = 0.01 * 512000
Bc = 5120
Formula No. 2 : PIR = CIR ( 1 + Be/Bc)
2048000 = 512000 (1 + Be/5120)
2048000 /512000 = 1+ Be/5120
4 - 1 = Be/5120
3 = Be/5120
Be = 3 * 5120
Be = 15360
Your "show policy-map interface" output proves your values.
Please rate if useful and question has been answered.
Thanks
12-10-2012 09:05 AM
By trial and error I arrived at the settings below. This was complete guess work:
policy-map SHAPE
class class-default
shape peak 512000 256000 768000
The idea was to set the target rate (cir) to 512000. ok so far.
Next, I know that PIR is 2048000 so I take that and divide by the default .125ms timing interval to arrive at bc 256000.
Lastly (now really on rocky ground) for the be value I played with the numbers until the show policy-map returned the correct numbers.
Can someone help me understand please, as for one I have no clue if this is correct, and 2) no clue about the method. I have to repeat the same for different bandwidths so really key I get this right.
it would be much appreciated.
R1(config-pmap-c)#do sh policy-map int f0/0
FastEthernet0/0
Service-policy output: SHAPE
Class-map: class-default (match-any)
39 packets, 4314 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
Traffic Shaping
Target/Average Byte Sustain Excess Interval Increment
Rate Limit bits/int bits/int (ms) (bytes)
2048000/512000 128000 256000 768000 500 128000
Adapt Queue Packets Bytes Packets Bytes Shaping
Active Depth Delayed Delayed Active
- 0 39 4314 0 0 no
12-10-2012 11:27 AM
Cisco routers use following formulas to calculate CIR (Commited Information Rate/Average Rate), Bc (Commited Burst/Sustain bits), Be (Excess Burt/Excess Bits), PIR (Peak Information Rate/Target Rate):
Tc = Bc/CIR
(Tc is time interval in ms)
=================================
peak rate i.e PIR = CIR(1+Be/Bc)
=================================
Please compute the values from your example and let me know.
Note: (EDITED):
You have a 500ms interval which could be a problem for a voice traffic so you can use a interval of 25ms by using following values:(just an example use any of your choice)
shape peak 512000 12800 38400
R1#show policy-map int fa0/0
FastEthernet0/0
Service-policy output: SHAPE
Class-map: class-default (match-any)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
Traffic Shaping
Target/Average Byte Sustain Excess Interval Increment
Rate Limit bits/int bits/int (ms) (bytes)
2048000/512000 6400 12800 38400 25 6400
Adapt Queue Packets Bytes Packets Bytes Shaping
Active Depth Delayed Delayed Active
- 0 0 0 0 0 no
R1#
12-10-2012 12:56 PM
well, according to my original configuration
tc = bc/cir
tc = 256000/512000
tc = 500 msecs
yes, you are right, and yes this will carry voice so that is going to be a problem.
PIR = CIR(1+Be/Bc)
PIR = CIR(1+ 768000/256000)
PIR = CIR * (1 + 3)
PIR = 512000 * 4
PIR = 2048000
the numbers add up.
Your suggestion "shape peak 512000 12800 38400"
tc = 12800/512000
tc = 25 msecs
PIR = 512000 * (1 + 38400/12800)
PIR = 512000 * 4
PIR = 2048000
So this is I guess my question, how to calculate be and bc values if tc, cir and pir are known (I never was any good at re-arranging formulas).
so first to calculate bc:
bc = tc * cir
bc = 0.025 seconds * cir
bc = 12800
...then to calculate be is simply ?
be = 3 * bc
be = 38400
not sure that is correct. any advice?
really appreciated Nandan, thanks
12-10-2012 01:09 PM
in the end, I went for a 10ms tc....the above calculations seems to carry......but I'd be interested to hear the 'correct way', this seems like a bit of a hack
cheers!!!!
R1(config-pmap-c)#do sh policy-map int f0/0
FastEthernet0/0
Service-policy output: SHAPE
Class-map: class-default (match-any)
1745 packets, 174762 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
Traffic Shaping
Target/Average Byte Sustain Excess Interval Increment
Rate Limit bits/int bits/int (ms) (bytes)
2048000/512000 2560 5120 15360 10 2560
Adapt Queue Packets Bytes Packets Bytes Shaping
Active Depth Delayed Delayed Active
- 0 1745 174762 0 0 no
12-10-2012 02:54 PM
I think I got it, can you comment to re-assure me please?
be = (PIR * tc) - bc
bc = tc * cir
12-10-2012 06:31 PM
Hi Philip,
Yes this is perfect.
Formula no.1 : Tc = Bc/CIR
10ms = Bc/512000
Bc = 0.01 * 512000
Bc = 5120
Formula No. 2 : PIR = CIR ( 1 + Be/Bc)
2048000 = 512000 (1 + Be/5120)
2048000 /512000 = 1+ Be/5120
4 - 1 = Be/5120
3 = Be/5120
Be = 3 * 5120
Be = 15360
Your "show policy-map interface" output proves your values.
Please rate if useful and question has been answered.
Thanks
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