Acceptable Load on WAN Link

Discussion in 'Cisco' started by ciscoham, Oct 17, 2005.

  1. ciscoham

    ciscoham Guest


    We have a 100 Mbps Leased Line between two Cisco 7301 routers. The load
    on the line during the normal office hours is around 40 % but at the
    start of the day the load of the line climbs to a peak that sometimes
    goes up to 100 %, mostly it doesn't get any higher then 90 %.
    All traffic on the line is data ( no Voice or Video ) and the
    applications are non-interactive like Lotus notes mail and other
    database applications.
    Now the customer is saying that loads above 60 % are unacceptable for
    them, I really think it would be a waste of money to use no more then
    60 % of the available bandwidth.
    I agree that having 100 % load is not good and was thinking on
    implementing RED on the routers to avoid this, but what I really need
    is a document that handles this topic and shows that the 60 % load is
    really unrealistic.
    Any documents with 'rules of thumb' or best practice are more then
    welcome, I would also like to hear your personal opinion on this.

    Thanks for your feedback
    ciscoham, Oct 17, 2005
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  2. What you are looking for is a review of queueing theory. Bottom line
    is that you can expect typical delay to increase dramatically as
    loading goes up. Assuming the delay for a single packet is X msec,
    the delay at 50% load will be 2 X, at 75% load it will be 4 X, at
    90% load expect 10 X and at 100% load, you're approaching infinity.

    The classical rule of thumb is to use a number between 60 and 70 %
    as the maximum design load for this reason. Good designers not
    only know where the numbers come from, but also know that they
    need to keep the numbers in perspective. If the zero load delay
    is barely acceptable for the application, 50% loading could be too
    heavy, while if the network provides 1 ms, and the application only
    requires 100 ms, anything less than 100% loading is fine.

    Bottom line, the answer to "how much load is too much load" is
    the same as the answer to almost all network design questions:
    "It depends..."

    Good luck and have fun!
    Vincent C Jones, Oct 17, 2005
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  3. ciscoham

    anybody43 Guest

    Ciscoham said:
    Vincent said:
    Vincent knows more than I do about this however
    the following has occurred to me.

    How about measuring the network delay
    directly and basing decisions on that?

    e.g. use some tool to ping across the link and
    report the results. Cisco SAA for example.

    I guess that a problem with this is that the
    delay measurement is likely to be a small sample
    of the traffic and the resulting conclusions
    will be subject to statistical uncertainty.

    Why are there no tools that sort all of this out for us?
    anybody43, Oct 17, 2005
  4. ciscoham

    ciscoham Guest

    Thanks a lot for your feedback, this gives me some stuff to think
    about. I'll do some reading on the www and get back later with more

    ciscoham, Oct 20, 2005
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