Be usage

hello

guys i am always confused with the Be how it works..

for example, Cir = 512000, Bc = 5120 and Be = 5120..

The cir is the maximum the link is going to support and traffic more then this will be shaped or policed (depends which on is configured)..

as per my understanding if there are some (tc) intervals left from Bc then the Be can use then..

Comments

  • Well, I'm no authority on QoS but here's my understanding of it... :)

     

    Access Rate (AR) is the maximum that a link will support.  Committed Information Rate (CIR) is what is contracted or guaranteed through the service provider network.  An example would be a DS1 carrier to the customer premise, with a 512kbps frame relay PVC riding over the DS1 circuit.

     

    So, using the numbers from your example:

     

    AR  = 1536000 bps

    CIR = 512000 bps

    Bc  = 5120

    Tc  = 10 ms

    Be  = 5120

     

    The Be value indicates burst per interval, above the CIR value.  In your case, up to 1024 Mbps.  Here's a quote directly from the DocCD -- or whatever it's called these days.

     

     

    "One additional variable applies to traffic shaping: excess burst (Be) size. The Be size corresponds to the number of noncommitted bits—those outside the CIR—that are still accepted by the Frame Relay switch but marked as discard eligible (DE).

     

    In other words, the Be size allows more than the burst size to be sent during a time interval in certain situations. The switch will allow the packets belonging to the excess burst to go through but it will mark them by setting the DE bit. Whether the packets are sent depends on how the switch is configured.

     

    When the Be size equals 0, the interface sends no more than the burst size every interval, achieving an average rate no higher than the mean rate. However, when the Be size is greater than 0, the interface can send as many as Bc plus Be bits in a burst, if in a previous time period the maximum amount was not sent. Whenever less than the burst size is sent during an interval, the remaining number of bits, up to the Be size, can be used to send more than the burst size in a later interval."

     

  • Typo error -- "In your case, up to 1024 Mbps" should read kilobits/second.

  • CIR is your target rate.  AR is the total link speed.

    Bc is your bits per time interval to achieve CIR.  (CIR/Tc = Bc)  in this case, your timing interval is 1/100 of a second.  So 5120 bits per 10ms --> 512k per second.

    Be is an optional and separate value.  You can't exceed whatever the difference between AR and CIR is (leftover unallocated potential transmission speed), but you can set whatever you want.  In this case, you allocated another possible 512Kbps (5120 bits per 1/100 second).

    HTH,

    Scott







    ----- Original Message -----
    From: "Bored" <[email protected]>
    Sent: Mon, January 26, 2009 11:24
    Subject: [CCIE R&S] Be usage


    hello

    guys i am always confused with the Be how it works..

    for example, Cir = 512000, Bc = 5120 and Be = 5120..

    The cir is the maximum the link is going to support and traffic more then this will be shaped or policed (depends which on is configured)..

    as per my understanding if there are some (tc) intervals left from Bc then the Be can use then..



    --
    View this message online at: http://ieoc.com/forums/p/4928/16111.aspx#16111
    --
    Internetwork Expert - The Industry Leader in CCIE Preparation

    http://www.internetworkexpert.com



    Subscription information may be found at:

    http://www.ieoc.com/forums/ForumSubscriptions.aspx
    <!>

  • Hello everyone,

    So be are the bits that are going in excess of the Cir...
    but the real different thing is in which time interval we will consider using the Be..

    for example our Tc = 10 msec that mean we have divided the second in to 10 equal intervals. lets say we are sending some data and the our Bc bits for that particular 10 msec is consumed but some portion of the data is still waiting to be sent, so will we use the Be bits here and send the data keepinig in mind that the SP will mark it as DE eligible..

    or we have to wait for some empty slots from Bc to use the Be facility.
    This usage of Be is a little confusing.

    moreover if we can use the Be regardless of the Tc meaning that we can use it whenever we want to then this means that our Cir will not be maintained and it will be Bc+Be in that particular time interval..

    Thanks

     

  • for example our Tc = 10 msec that mean we have divided the second in to 10 equal intervals. lets say we are sending some data and the our Bc bits for that particular 10 msec is consumed but some portion of the data is still waiting to be sent, so will we use the Be bits here and send the data keepinig in mind that the SP will mark it as DE eligible..

     

    If you have accumilated credit (tokens in token bucket), only then Be will be used. Lets say you are given a circuit of 512K with a CIR of 256, your average rate will be 256 but if you have empty 'time slots' (The amount of data being sent is less than CIR), you can configure Be to transmit excess data bits upto AIR.

    Be is only useful if your CIR is less than AIR and you have credit.

    Here is a good brief explanation.

    http://www.cisco.com/en/US/tech/tk713/tk237/technologies_configuration_example09186a00800942f8.shtml

  • You divided into 100 equal intervals.  10ms is 10/1000 seconds or 1/100th.

    The DE elegible part will come a little differently, because the SP is measuring slightly differntly than what your shaper is.  but general concept, yes.

    Emtpy slots from Bc are still allocated to Be.  be bits are "above" bc in the same interval.

    HTH,

    Scott







    ----- Original Message -----
    From: "Bored" <[email protected]>
    Sent: Tue, January 27, 2009 9:10
    Subject: Re: [CCIE R&S] Be usage


    Hello everyone,

    So be are the bits that are going in excess of the Cir...
    but the real different thing is in which time interval we will consider using the Be..

    for example our Tc = 10 msec that mean we have divided the second in to 10 equal intervals. lets say we are sending some data and the our Bc bits for that particular 10 msec is consumed but some portion of the data is still waiting to be sent, so will we use the Be bits here and send the data keepinig in mind that the SP will mark it as DE eligible..

    or we have to wait for some empty slots from Bc to use the Be facility.
    This usage of Be is a little confusing.

    moreover if we can use the Be regardless of the Tc meaning that we can use it whenever we want to then this means that our Cir will not be maintained and it will be Bc+Be in that particular time interval..

    Thanks

     



    --
    View this message online at: http://ieoc.com/forums/p/4928/16174.aspx#16174
    --
    Internetwork Expert - The Industry Leader in CCIE Preparation

    http://www.internetworkexpert.com



    Subscription information may be found at:

    http://www.ieoc.com/forums/ForumSubscriptions.aspx
    <!>

Sign In or Register to comment.