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Traffic Shaping. Understanding auto generation of Tc/Bc values

adamclarkuk_2
Level 4
Level 4

Hi All

My question relates to the auto generation of the Tc / Be value when using traffic shaping

within IOS and class based shaping. I am using the command :-

Shape average x

The formula is as below :-

Tc = Bc / CIR

Bc = Tc * CIR

I came across some Cisco documentation that says that the Bc will be 1/8 of the CIR by default therefore giving a Tc value of 125. I also understand that the minimum value for Tc must be =>10 (unless using DTS on the 7500) and <=125.

The majority of my testing has been on a 7206 VXR running 12.4, on Ethernet/Fastethernet/Serial interfaces and none of the above seems to be true.

Ethernet and Fast Ethernet seem to use a Tc value of either 25ms or sometimes 24ms no matter what I set the CIR value to, and the serial Tc value is all over the place.

I can force a Tc value by specifying the Bc as an argument in the shape average command, but should I be doing this?

I guess my questions are as followg :

1. Can I calculate the Bc and Tc values using a formula before I enter the shape average command, or do I need to enter the command then type show policy-map interface x

2. How is IOS calculating the values by default?

1 Accepted Solution

Accepted Solutions

Joseph W. Doherty
Hall of Fame
Hall of Fame

#2 I believe it varies per platform and IOS. I believe I've noticed the latest 12.4T default is 4ms(?).

(NB: Also on the latest IOSs, believe there are new options for specification of parameters. One, I believe, now supports a specification in milliseconds.)

#1 Yes, you can precalculate. If I remember correctly, if you divide the second or third parameter (Bx in bits) by the first paramter (CIR in bps), it will provide the Tc (i.e. Tc = Bx/CIR - your first formula).

Assuming you want a desired Tc (and the IOS doesn't support the parameter is ms), use your 2nd formula, Bc = Tc * CIR.

e.g.

given a desire for 15 milliseconds and 2.5 Mbps

Bc = .015 * 2500000 = 37500

PS:

I'm not at a Cisco device, at the moment, to confirm #1 answer. If I'm incorrect, let me know. (I might be missing some bits vs. bytes or confusing what parameter represents what, but you can precalculate.)

View solution in original post

2 Replies 2

Joseph W. Doherty
Hall of Fame
Hall of Fame

#2 I believe it varies per platform and IOS. I believe I've noticed the latest 12.4T default is 4ms(?).

(NB: Also on the latest IOSs, believe there are new options for specification of parameters. One, I believe, now supports a specification in milliseconds.)

#1 Yes, you can precalculate. If I remember correctly, if you divide the second or third parameter (Bx in bits) by the first paramter (CIR in bps), it will provide the Tc (i.e. Tc = Bx/CIR - your first formula).

Assuming you want a desired Tc (and the IOS doesn't support the parameter is ms), use your 2nd formula, Bc = Tc * CIR.

e.g.

given a desire for 15 milliseconds and 2.5 Mbps

Bc = .015 * 2500000 = 37500

PS:

I'm not at a Cisco device, at the moment, to confirm #1 answer. If I'm incorrect, let me know. (I might be missing some bits vs. bytes or confusing what parameter represents what, but you can precalculate.)

Hi Joseph

Nice one mate, it looks like it was just the IOS I was using for testing. 12.4T always comes back with 4ms for every value I enter. I also noticed the output of 'show policy-map interface x' has changed, it doesn't show you the Tc value anymore, I had o break out the calc ;-).

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