Cisco 3550 resetting TOS bits?

Discussion in 'Cisco' started by foldingwidgets-cisco@yahoo.com, Apr 13, 2005.

  1. Guest

    As part of some product testing that I am doing, I am sending traffice
    between two end nodes where the some of the traffic has the
    Minimize-delay TOS bit set and some has the Maximize-throughput bit
    set. If I sniff the packets on the originating side, the IP header is
    what I expect. However on the other end (after passing through a 3550
    switch) I see that the TOS bits have all been reset to 0x00.

    Is this normal operation? Is there a way to disable this?

    TIA!
    , Apr 13, 2005
    #1
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  2. In article <>,
    <> wrote:
    :As part of some product testing that I am doing, I am sending traffice
    :between two end nodes where the some of the traffic has the
    :Minimize-delay TOS bit set and some has the Maximize-throughput bit
    :set. If I sniff the packets on the originating side, the IP header is
    :what I expect. However on the other end (after passing through a 3550
    :switch) I see that the TOS bits have all been reset to 0x00.

    :Is this normal operation? Is there a way to disable this?

    As I recall, the TOS bits are one of the criteria used for QoS
    classification on the 3550. The output 0x00 could represent the
    result of QoS -- that is, everything is likely being mapped to the
    same default priority.

    I have read a little about QoS but never played with it, so I do not
    know whether this could be disabled. -Possibly- there is a method
    involving mapping the various interesting ToS bits into the corresponding
    binary-number priority. But the 3550 does not have 8 different
    output queues, so you might be forced to map more than one point to
    the same output ToS I guess. (The 3750 has a greater number of
    output queues if I recall correctly.)

    --
    Studies show that the average reader ignores 106% of all statistics
    they see in .signatures.
    Walter Roberson, Apr 13, 2005
    #2
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  3. Scooby Guest

    <> wrote in message
    news:...
    > As part of some product testing that I am doing, I am sending traffice
    > between two end nodes where the some of the traffic has the
    > Minimize-delay TOS bit set and some has the Maximize-throughput bit
    > set. If I sniff the packets on the originating side, the IP header is
    > what I expect. However on the other end (after passing through a 3550
    > switch) I see that the TOS bits have all been reset to 0x00.
    >
    > Is this normal operation? Is there a way to disable this?
    >
    > TIA!
    >


    It is normal. By default, your equipment will not accept incoming QOS - for
    good reason. On the incoming interface type:

    mls qos trust cos

    that will tell the interface that your equipment trusts the QOS coming in.

    Hope that helps,

    Jim
    Scooby, Apr 14, 2005
    #3
  4. Guest

    Thank you so much! (I actually had to use "mls qos trust cos
    pass-through dscp").
    , Apr 15, 2005
    #4
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