what does "delay" mean?

Discussion in 'Cisco' started by Weiguang Shi, Nov 6, 2003.

  1. Weiguang Shi

    Weiguang Shi Guest

    Hi,

    When I do a "show interface se0/0", among other things, I get this line

    MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec

    As I understand, MTU is a value which one can change to whatever he likes
    (correct me if I am wrong), but the BW and DLY seem a little confusing.

    I read somewhere that BW is there to give upper-layer software a hint and does
    not have to equal transfer rate (clock rate). On a router, the upper layer
    software is IP, which I don't recall needs a BW parameter to run efficiently,
    let alone an inaccurate one.

    The DLY seems more blurred. It's certainly not propagation delay:

    20000 usec * Speed-of-Light * 2/3 = 4000km

    but my cable is at most 5 meters. Is it serialization delay (the time taken to
    sent one bit)?

    1/1544000 = 0.65 usec

    So neither. What is it then?

    Thanks,
    Wei
     
    Weiguang Shi, Nov 6, 2003
    #1
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  2. :When I do a "show interface se0/0", among other things, I get this line

    : MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec

    :As I understand, MTU is a value which one can change to whatever he likes
    :(correct me if I am wrong), but the BW and DLY seem a little confusing.

    :I read somewhere that BW is there to give upper-layer software a hint and does
    :not have to equal transfer rate (clock rate). On a router, the upper layer
    :software is IP, which I don't recall needs a BW parameter to run efficiently,
    :let alone an inaccurate one.

    BW is used by routing metrics, with the cost of the route being
    inversely proportional to the bandwidth.


    :The DLY seems more blurred. It's certainly not propagation delay:

    Good question. I do not see a good explanation. I can see in
    the examples that the delay goes down as the bandwidth goes up,
    but the relationship is approximate.
     
    Walter Roberson, Nov 6, 2003
    #2
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  3. Weiguang Shi

    shope Guest

    BW is also used by management systems which pull stats back from the network
    devices. If bandwidth isset correctly, an NMS can get the interface
    bandwidth from each device and give you %age load on each interface, warn of
    congestion etc.

    BW also gets used to set some QoS parameter - eg. i think that RSVP uses it
    to device on whether reservations are allowed over a path.

    So - not a good parameter to play with to fix routing issues.
    Delay is another magic number used by a routing protocol to decide on the
    "best" route for traffic. It was intended to be the "delay" for packets over
    that path, so a routing protocol could choose the lowest delay path to send
    packets.

    Like a lot of networking parameters, it doenst seem to mean that much
    anymore, but the name is still the same to confuse everyone.

    The main protocol that uses this on a cisco router tends to be EIGRP - you
    can alter this to bias a routing protocol to choose different paths - i
    often increase delay values on an ISDN backup interface to stop traffic
    flows using ISDN in preference to a fixed link.

    If you want more detail you really need to look at a description of route
    choices in EIGRP - see
    http://www.cisco.com/en/US/tech/tk365/tk207/technologies_tech_note09186a00800c2d96.shtml
     
    shope, Nov 6, 2003
    #3
  4. Weiguang Shi

    Weiguang Shi Guest

    Thank you both!

    Wei
     
    Weiguang Shi, Nov 6, 2003
    #4
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