Mixing jumbo and standard frames?

Discussion in 'Linux Networking' started by Roger Blake, Nov 15, 2007.

  1. Roger Blake

    Roger Blake Guest

    I'm looking at upgrading a Linux-based network to gigabit speed using 9K
    jumbo ethernet frames. However, there are still some old 100Mbit computers
    and devices that will still need to be connected to the network, these
    needless to say do not support jumbo frame sizes.

    It is my understanding that everything on the LAN needs to have the
    same ethernet frame size. What is the best way to connect the jumbo and
    non-jumbo devices? Web searching indicates that 802.1Q VLANs may
    be the answer, but there is a lot of information to dig through and the
    implementation details are not clear to me.

    Any pointers on how to do this would be greatly appreciated - thanks!
    Roger Blake, Nov 15, 2007
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  2. Roger Blake

    Rick Jones Guest

    If all your traffic is TCP, then you can "get away" with setting Jumbo
    Frames on a subset of the systems. The reason is the TCP MSS (Maximum
    Segment Size) exchange at the beginning of each connection. The
    systems without JF enabled will specify a smaller MSS and all will be

    The issue arrises with UDP or direct link-level stuff, or anything
    else without a TCP-like MSS exchange. Normal stuff like DNS will be
    OK (IIRC) since that is already keeping itself below 1500 bytes
    (handwaving). NFS over UDP however will have problems if you try to
    go from a JF to non-JF system. The closest thing there to an MSS
    exchange is the mount size and crippling to a 1024 byte mount size
    would probably be worse than leaving the JF off.

    Otherwise, the two systems have to be in separate IP subnets, with a
    router in between them and/or you have to start adding a lot of host
    routes with specific PathMTU's set on each JF-enabled system.

    They don't necessarily have to be in separate VLANs. The "router" in
    this case would have to be a JF-capable system, _perhaps_ with two
    NICs but I think you can get away with logical interfaces instead - if
    you can assign separate MTU's to each logical interface (eth0 and
    eth0:1 in linux-like parlance). When JF talks to non-JF through the
    router, it will fragment the IP datagrams to the proper size.

    rick jones
    Rick Jones, Nov 15, 2007
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