Mar
23

This blog post is taken from the INE Resources area Understanding Frame-Relay Traffic Shaping presentation by Brian Dennis.

Overview

Frame-Relay traffic shaping is designed to control the amount of traffic the router sends out of an interface or out of a particular DLCI. Common reasons for Frame-Relay traffic shaping are:

  • It allows the router to conform to the rate subscribed with the service provider
  • It allows for the throttling of a higher speed site (768K) so that it does not overrun a lower speed site (64K)

Traffic shaping is designed to delay excess traffic, whereas policing is designed to drop excess traffic.

Terminology

  • Available Rate (AR) – the actual physical speed of the interface; on a DCE serial interface this is determined by the configured clock rate. On a DTE serial interface, it is determined by the received clock rate. A router will always (by default) try to send out at the AR regardless of the interface bandwidth. AR is also commonly referred to as port speed, line rate, or access rate.

  • Committed Information Rate (CIR) – the rate the device will send at (on average) over a one second period. The default CIR when traffic-shaping is enabled on the interface is 56K. CIR is also referred to as the “target rate”. Since the device is forced to send at the AR, it does not send all of the time (within one second) ┬áin order to send an average amount of data that equals the CIR.
  • Minimum CIR (mincir) – the rate the service provider guarantees to accept. Theoretically, the provider will set the DE bit for all traffic above this rate. Mincir is designed to be used in conjunction with adaptive shaping. With adaptive shaping, the router will throttle down in the event of congestion. The router will not throttle down below this value.
  • Committed Burst (Bc) – the number of committed bits allows to be sent during a given interval. The device sends an average amount of traffic to achieve the CIR. The Bc value defaults to 1/8 of the configured CIR for speeds below 650K. For speeds above that, it is roughly 1/16 of CIR.
  • Excess Burst (Be) – the number of non-committed bits the router is allowed to send above Bc during the first interval (Tc). The amount of Be “credits” is derived from unused Bc credits in previous intervals. There is no limit to how long Be can “store” unused Bc credits. It is a common misconception that Be can only store credits from the previous interval or the previous second. There is no default Be value.
  • Committed Rate Measurement Interval (Tc) – the time interval over whic Bc or Bc+Be can be transmitted. The max value is 125 ms and the minimum value is 10 ms.

The Formula

CIR, Tc, and Bc are related mathematically by the following formula:

CIR = Bc/(Tc/1000)

Notice the division of Tc by 1000 is used to convert milliseconds into seconds – the common measurement of CIR and Bc.

Example 1

Task:

The port speed of your serial interface is 192Kbps. You must shape the interface to send at 128Kbps.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 128000

Example 2

Task:

The port speed is 384K. The router should send at a rate of 192K. The router should throttle down to 128K upon the receipt of BECNs. Allow the router to burst up to port speed.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 192000
frame-relay be 24000
frame-relay mincir 128000
frame-relay adpative-shaping becn

Example 3

Task:

The port speed is 384K. The router should send at a rate of 128K. The router should throttle down to 64K upon the receipt of BECNs. Allow the router to burst up to port speed. Use a timing interval of 100 ms.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 128000
frame-relay bc 12800
frame-relay be 25600

frame-relay adpative-shaping becn

You can leave a response, or trackback from your own site.

13 Responses to “Legacy Frame-Relay Traffic Shaping Basics”

 
  1. Wai Yew Choy says:

    Hi Anthony,

    Thanks for the detail explanation…

    Like to know how you get 24000 in Be in Example 2:

    frame-relay be 24000

    Also the following figures in Example 3…
    frame-relay bc 12800
    frame-relay be 25600

    Thanks..

  2. Niels says:

    Wai Yew Choy, for the Example 3 question. These values are represent what can be sent in every Tc interval=100ms. 128000/0,1sec (100ms)=12800. this also goes for the Be (Exceed Burst), it is the amout of bits that can be sent in the excess interval, up to a max of 384Kbps which is the access rate, 12800+25600 per Tc interval (100ms).
    By default upon receipt of the BECN the source will throttle down to 64k unless the frame-relay mincir is specified.

    HTH

  3. Yasir Ashfaque says:

    It is calculated as EIR*TC so the formula is
    Port Speed == CIR+EIR

    BC = CIR*TC
    BE = EIR*TC

    so now the portspeed of 384K, we got CIR of 128K which makes BC = (128K)(100ms) == 12800

    and BE = (256K)(100ms) == 25600

    Also we did not use mincir in solution 3 because it by defaults drops down to half.

    @Anthony, correct me if im wrong :)

  4. Deepak Arora says:

    Hi Team,

    Can you please add options (download as pdf) for each blog post, because that can help us allot because we can download the tutorial and can read it offline with pdf reader.

    Thanks!
    Deepak Arora

  5. Yasir Ashfaque says:

    Deepak, its a nice idea.

  6. Adrian says:

    I could be mistaken but isn’t your formula wrong on the top?

    CIR = Bc/(Tc/1000)

    Shouldn’t it be

    Bc = CIR / Tc

  7. Marcos says:

    @Wai Yew,

    In example #2 the used formula is:

    Be = (AR – CIR) * 125/1000

    Which give us the 24000 value used by Anthony.
    Tc = 125 is the default value for every pvc in frame-relay (when traffic-shaping is enabled).

    Hope this helps.

    Cheers.

    Marcos.

  8. Leo says:

    Adrian, I think the formula is ok.
    CIR is in bits per seconds
    about Tc were speaking default in miliseconds,

    so this is just to have Tc in seconds

    Leo

  9. NET_OG says:

    A few math mistakes here (mainly in comments).

    Example 2:

    Anthony was solving for Bc not Be.
    The logic is like this
    1) we know default time interval Tc = 125msec or .125 sec
    2) We know that the CIR is 192000bps
    3) Solve the equation

    Tc = Bc/CIR
    when solving for Bc
    Bc = Tc * CIR
    Bc = .125sec * 192000bps = 24000bits

    Leo was right: By Default Tc is in MicroSeconds (ms) but CIR is given in Seconds so that is why the formula had the “1000″ in the denominator.

    Here it is again: CIR = Bc/(Tc/1000)

    I just convert straight off instead of playing games with fractions in fractions and trying to remember rules that only Petr remembers. :-)

    I had to read this a few times and get some scratch paper out to remind myself of the math. Once I did that it made more sense.

    BTW: one thing that is really annoying is when the Bc is listed as Xkbps. Bc is just a unit of bits.

    Great post. thanks guys. I just reviewed Frame-relay traffic shaping. I can check that off the list.

  10. NET_OG says:

    milliseconds Tc is measured in milliseconds

    ARGH! Sorry for the TYPO

  11. ccent beginner says:

    Hi,

    Seem like the question can be solved by different ways/methods,and everyone has it’s own opinions, i would like to share out my thought as well. Please bear with me if my thought is incorrect, as i just begin in “Networking”…

    Example 2:
    Task:

    The port speed is 384K. The router should send at a rate of 192K. The router should throttle down to 128K upon the receipt of BECNs. Allow the router to burst up to port speed.

    Based on the question given, we knew that:
    1.Port Speed is given as 384kbps;
    2.CIR is given as 192kbps;
    3.MinCIR is given as 128kbps.

    I would assume that aside the value given above, every other value should be based on it’s default. We all know that the formula, Tc = Bc/CIR. So i would figure out the Bc & Be value based on this formula “+” the IOS natural, what i mean is the “Bc” value.

    Many of us “assumed” that the value of Tc = 125ms/0.125s,i do agreed that,WHEN the value of CIR is equal to 56000bps or 56kbps,and this’s the “default CIR value” for CISCO 7200 and lower platform.In our question,it has already mentioned that the value of CIR = 192kbps,so the assumption of Tc=125ms is not valid at all.So what we going to do? We’ve to calculate the Tc value as well.

    Refer to CISCO Enterprise QoS SRDN page 3-26,section CIR. “By default, Cisco IOS Software set the Bc to CIR/8, for Data”.We got the formula of Bc = CIR/8, and we will get the figure Bc=24000bps, is that okay?

    Then back to the formula,Tc = Bc/CIR,we got now Bc=24000 & CIR=192000, and Tc will be 0.125 OR 125ms if it’s measured in milisec.

    So how about Be? Got some bro mentioned that Be=EIR x Tc. We got the EIR which is 192000 & Tc value which we’ve calculated. Be=192000×0.125, and we’ll get the desired value as Anthony shown to us.

    The methodology is quite confused and troublesome,but at least i can proved that how do i get every single value in the question.If any value not given, i will take software nature behaved into consideration.

    Again, above explanation just my personal view when preparing the CCENT.

    Source of FR configuration default value:
    http://www.cisco.com/en/US/tech/tk713/tk237/technologies_configuration_example09186a00800942f8.shtml

    Enterprise QoS SRND, FR design page 3-25.

    HTH

  12. NET_OG says:

    CCENT EB.

    great link!

    That’s why I keep coming back to this blog. By far the most friendly and collaborative students on the Internet. thanks for sharing the link and your alternate method.

  13. Fernando says:

    In Ex 3, we should have used :
    frame-relay mincir 64000

    So we can throttle according to BECNs frames, shouldn’t we??

 

Leave a Reply

Categories

CCIE Bloggers