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  1. #1
    Join Date
    Sep 2011
    Posts
    103

    Default Maximum MTU Sizes

    Hello folks

    Scenario:

    Users report latency towards one of their servers. You run ping test from a different site to your customer router WAN - WAN all ok | LAN to Your customers Router LAN interface all ok, now when pinging from customers router LAN interface towards this server you get packets loss 1 day with an MTU sizes of 1500, the next day the same trouble but with an MTU of 1500 max 2 packets drop out of 1000 sent, so you try with I believe we used 18000, can this sizes be use?

    That's my doubt if someone could clarify the maximum or how to determine the maximum packet sizes for a specific device or service.


    Thank you in advance
    Last edited by n3twrk0p; 02-24-2012 at 08:26 AM.

  2. #2
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,441

    Default

    To support jumbo frames (up to 9000 bytes) you need to be using 1Gbps links. An MTU of 1500 is currently the standard MTU size for the Internet. Super jumbo frames (over 9kb) would require faster-than-1Gbps connections, such as 10GbE, although this isn't a current standard.

    Some WAN links require you to drop the MTU to 14xx so that packets are not fragmented by lower bandwidth requirments. If there are packet losses over the WAN, it's better to reduce the MTU rather than increase it.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  3. #3
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    Any drops on the interface mate?
    I would guess that a time of the day the link is getting over used if at some parts it is ok and other it is not.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  4. #4
    Join Date
    Sep 2011
    Posts
    103

    Default

    Thank you for your inputs gentlemen,

    Yeah we ran a ping test over 1500 in sizes can't recall the bandwidth of the link but i do know it was not a Gb. The client was complaining about slow performance during normal operation hours, it could be what BE was stating, link over utilization. In case it happens again they will want to set up another 3 hour conference call for nothing cause the trouble is on their side (LAN) not on ours...anyway will definitely pay close attention to the bandwidth been use if we see any packet loss.

    Fuzz is there a chart that shows this jumbo MTU sizes where it explains there usage?

    again, thank you

  5. #5
    Join Date
    Sep 2011
    Posts
    103

    Default

    I was just checking my mail now and noticed that one of our techs was able to identify the trouble causing the slow performance, the link from the router to the switch was negotiating at 100Mbps when it was suppose to be 1Gb there you go BE right on the spot .

  6. #6
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,441

    Default

    Quote Originally Posted by n3twrk0p View Post
    Fuzz is there a chart that shows this jumbo MTU sizes where it explains there usage?
    Where you would use jumbo frames is generally where you expect high volumes of traffic between two devices. We use it within our iSCSI network where large volumes of data get written and read to/from the SAN.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  7. #7
    Join Date
    Sep 2011
    Posts
    103

    Default

    Quote Originally Posted by Fuzz View Post
    Where you would use jumbo frames is generally where you expect high volumes of traffic between two devices. We use it within our iSCSI network where large volumes of data get written and read to/from the SAN.

    I suppose in Voice traffic as well, got it now.


    Thank you Fuzz

  8. #8
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    Quote Originally Posted by n3twrk0p View Post
    I was just checking my mail now and noticed that one of our techs was able to identify the trouble causing the slow performance, the link from the router to the switch was negotiating at 100Mbps when it was suppose to be 1Gb there you go BE right on the spot .
    LOL - i do know something then!
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

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