t3 MTU size versus a 100MBps MTU...

Discussion in 'Cisco' started by Bill B., May 4, 2004.

  1. Bill B.

    Bill B. Guest

    Hello,
    Today we have a T3 link that connects two buildings together. We are
    currently using an MTU of 4352 for this particular link. I am considering
    replacing this WAN T3 link with a true 100Mbps Ethernet link (Which would
    now set my MTU size down to ~1500) to connect the same two sites. Currently
    our T3 link is being utilized no more the 45% at any one time. The benefits
    of having a higher MTU, as I understand it, is that more stations could talk
    at once... or better put, instead of everyone standing in one long line to
    make it across the WAN link, there are multiple lines (I guess 4:1 or 3:1)
    that can be used. We currently have Catalyst 6006s on both sides. I guess my
    questions are:

    - Am I describing the benefits of having a higher MTU correctly?

    - Should I expect Higher Latency, and more bandwidth? Or Same Latency and
    more bandwidth?

    - Are my buffers for the Ethernet ports going to possibly overload with
    these 3 or 4 lines being squeezed into one?

    - Should I do this <grin>?

    - Where should I look for information into this?

    Thank you in advance...

    Bill Bushong
     
    Bill B., May 4, 2004
    #1
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  2. : Today we have a T3 link that connects two buildings together. We are
    :currently using an MTU of 4352 for this particular link. I am considering
    :replacing this WAN T3 link with a true 100Mbps Ethernet link (Which would
    :now set my MTU size down to ~1500) to connect the same two sites. Currently
    :eek:ur T3 link is being utilized no more the 45% at any one time. The benefits
    :eek:f having a higher MTU, as I understand it, is that more stations could talk
    :at once... or better put, instead of everyone standing in one long line to
    :make it across the WAN link, there are multiple lines (I guess 4:1 or 3:1)
    :that can be used.

    :- Am I describing the benefits of having a higher MTU correctly?

    Not even close.

    When there is a large burst of data to send (e.g., ftp, but not
    telnet or NFS version 1) then having a large MTU allows more of that
    data to be sent at one time. In some cases, that can make a big difference
    for performance, as you eliminate the packet-header overhead and
    intra-packet gap times and reduce fragmenting.

    If there does not happen to be large bursts of data to send, then
    the longer MTU makes no difference.

    When a large burst of data is sent by virtue of the larger MTU, then
    everyone else who is trying to send data has to wait longer for
    their turn, because it takes longer to send the larger packet over the link.
    Especially if the large packets tend to come from one particular source
    and not the other sources, that -increases- latency for everyone else.

    Whether you should expect lower latency or not when you go to
    an MTU of 1500 is not a simple question. It depends on how your
    link is being used. For example, if those large packets are not
    sent often, but there a lot of smaller packets, then sending that
    large packet in a burst might then allow that particular sender
    to go quiet, leaving less contention for the line, and less collisions.
    With the smaller MTU, then that large packet would have to be
    fragmented, potentially leading to increased collisions as that
    sender tries to gain primacy on the link. But the timings are
    crucial to the behaviour -- if you research the topic of the
    "ethernet capture effect" you will see some hints of how the
    exact packet timings can make a big difference in how different
    systems are granted access to the line.


    You don't happen to mention whether the new line will be full duplex or
    half duplex. Switches and a full duplex link should result in orderly
    queuing and buffering rather than in the kind of contention scenarios I
    describe in the previous paragraph. The capture effect occurs only
    with shared collision domains, which doesn't happen if you are fully
    switched, every NIC going directly to a switch port. The capture effect
    -can- occur if you have hubs, or if you have a shared media such as
    thinwire instead of Cat5.
     
    Walter Roberson, May 4, 2004
    #2
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  3. However, if the MTU of the inter-building link is larger than the MTUs
    of the LANs within the buildings, the extra MTU will never be used.
    Routers don't ever combine frames, so if the devices on the LANs never
    send anything larger than 1500 bytes, the frames sent over the T3 will
    never be larger, either.
     
    Barry Margolin, May 4, 2004
    #3
  4. Bill B.

    Captain Guest


    Interesting point!

    I use a MTU of 1500 thoughout my network, but most users connect
    to me through dsl connections,(terminating on a FastEthernet card on
    cisco 3640 box).

    I heard that adsl use a smaller MTU.

    Should I lower the MTU on my 3640,(and thoughout my network)?
     
    Captain, May 12, 2004
    #4
  5. :I use a MTU of 1500 thoughout my network, but most users connect
    :to me through dsl connections,(terminating on a FastEthernet card on
    :cisco 3640 box).

    :I heard that adsl use a smaller MTU.

    Often use, but not always. Consumer-level ADSL often uses PPPoE, but
    business level doesn't always, especially for SDSL.

    :Should I lower the MTU on my 3640,(and thoughout my network)?

    If you never use the jumbo packets, you might as well lower the MTU
    to long enough to support tagged (possibly MLPS) frames, but there's no
    point in going any higher. Just confuses people about what's going on
    over the link. But other than the confusion, it doesn't really hurt.
    (But it might potentially get you into a different pricing model to drop
    down to MTU ~ 1500, as that opens up more technologies.)
     
    Walter Roberson, May 12, 2004
    #5
  6. Bill B.

    Captain Guest



    Yes, we are using PPPoE,(I now it's smaller then 1500, but I'm not
    sure what it is exactly)?

    Is there any standard out there?
     
    Captain, May 12, 2004
    #6
  7. |On 12 May 2004 21:41:41 GMT, -cnrc.gc.ca (Walter
    |Roberson) wrote:

    |>In article <>,

    |>:I heard that adsl use a smaller MTU.

    |>Often use, but not always. Consumer-level ADSL often uses PPPoE, but
    |>business level doesn't always, especially for SDSL.

    |Yes, we are using PPPoE,(I now it's smaller then 1500, but I'm not
    |sure what it is exactly)?

    |Is there any standard out there?

    Not really. 1492 is common, but it depends on the ISP. I've seen 1450
    as common, and I've seen MTUs as low as 1392 for some of them.

    My residential ISP used to use 1476, but now allows higher.
     
    Walter Roberson, May 12, 2004
    #7
  8. Bill B.

    Captain Guest


    1472 looks like the size that our local cable company uses.
     
    Captain, May 13, 2004
    #8
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