available bandwidth

Discussion in 'Cisco' started by srp336, Mar 1, 2006.

  1. srp336

    srp336 Guest

    Thanks to everyone who responded to my recent posts regarding setting
    up EIGRP unequal cost routing. The load balancing is looking pretty
    good these days...

    In doing some speed tests over these T1 lines in response to some
    concerns we had here over throughput, I noticed something on one of the
    interfaces. Under 'output queue', it shows 'available bandwidth' and a
    number that's 75% of the stated bandwidth. Is this 75% figure standard?
    Can it be adjusted?

    My main concern is being able to fully explain to management how much
    bandwidth we can expect to be getting through these T1s...

    srp336, Mar 1, 2006
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  2. srp336

    ciscodagama Guest

    Yes, that is the default and it indicates that the maximum reservable
    bandwidth on the interface. The reservation is done when QoS is
    configured on the system. You can change that using the
    "max-reserved-bandwidth" command under the interface.
    The idea with only allowing 75% of the bandwidth to be reservable by
    default is to allow 25% for routing protocols and other such important
    traffic. BTW since you are running EIGRP, you can also look into a
    command that limits how much of the bandwidth EIGRP is allowed to use.
    That command is "ip bandwidth-percent eigrp percent ".

    Cisco da Gama
    ciscodagama, Mar 1, 2006
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  3. srp336

    srp336 Guest

    I see... I can understand setting aside some of the bandwidth for
    overhead, but isn't 25% a little much? In case, that comes to about 600
    kbit/s. The only routing protocol going over these T1s is EIGRP with a
    handful of networks.

    Are there any kind of guidelines to adjusting this? Would it be better
    to use max-reserved-bandwidth, or ip bandwidth-percent eigrp?

    srp336, Mar 2, 2006
  4. srp336

    ciscodagama Guest

    Yes, that is the default and it indicates that the maximum reservable
    Note that 75% number is just the max reservable bandwidth when you are
    using QoS. It does not mean that only 75% of the bandwidth will be
    used. If there is user traffic, it will use the all the bandwidth it
    can if QoS is not configured. It is just that you cannot use QoS to
    give bandwidth guarantees beyond 75% if the interface bandwidth unless
    you increase the reservable bandwidth using the
    "max-reserved-bandwidth" command.
    By default, EIGRP will use no more than 50% of the link bandwidth. If
    you have an unstable network where you think EIGRP is using too much of
    your link bandwidth, you can limit that further using the "ip
    bandwidth-percent eigrp" command to less than 50%. If the network is
    stable, I would suggest just leaving that alone.

    Cisco da Gama
    ciscodagama, Mar 2, 2006
  5. srp336

    srp336 Guest

    I see... there seems this ceiling at 75% of the bandwidth that we
    can't seem to get through. I don't think I've ever configured any QoS
    before. Could something be on by default?

    The T1s in question are a full T1 and 14 channels of another one. I
    expect the theorectical maximum bandwidth to be 2432 kbit/s (38
    channels X 64 kbit/s/channel). The peak speed showing up on our router
    stats is about 1800 kbit/s, about 75% (that was a location to location
    backup at around 4am when the rest of the network was quiet). During
    the day, we seem to get an average throughput of 1200 to 1400 kbit/s
    in the tests that I've done. I don't see any points that seem to be

    Is the throughput we're getting reasonable for the bandwidth we have?

    srp336, Mar 2, 2006
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