EIGRP, GRE and MTU

Discussion in 'Cisco' started by Andrey Tarasov, Dec 18, 2004.

  1. Hello, All!

    I found some strange thing and wonder if somebody seen it before. I have a Cisco
    2651 router with 12.2(24) IOS running EIGRP. There is GRE tunnel to remote site.
    Everything is working except from the fact that EIGRP updates gets fragmented by
    GRE. Changing "ip mtu" doesn't do any difference except the case when "ip mtu"
    set to 1500, than it's GRE packet which gets fragmented. I'm not sure if EIGRP
    tracks "ip mtu" and just miscalculate packet size or ignores it altogether.

    I have a case opened with Cisco but all I've got so far is two statements -
    EIGRP pays attention to MTU and EIGRP doesn't do that.

    With best regards,
    Andrey.
    Andrey Tarasov, Dec 18, 2004
    #1
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  2. Andrey Tarasov

    Hansang Bae Guest

    On Fri, 17 Dec 2004 22:41:19 -0800, "Andrey Tarasov"
    <> wrote:

    >Hello, All!
    >
    >I found some strange thing and wonder if somebody seen it before. I have a Cisco
    >2651 router with 12.2(24) IOS running EIGRP. There is GRE tunnel to remote site.
    >Everything is working except from the fact that EIGRP updates gets fragmented by
    >GRE. Changing "ip mtu" doesn't do any difference except the case when "ip mtu"
    >set to 1500, than it's GRE packet which gets fragmented. I'm not sure if EIGRP
    >tracks "ip mtu" and just miscalculate packet size or ignores it altogether.
    >
    >I have a case opened with Cisco but all I've got so far is two statements -
    >EIGRP pays attention to MTU and EIGRP doesn't do that.
    >


    I haven't seen this yet but I guess we might run into it as we are
    moving to 12.2.24a for quite a few of the branch routers (but OSPF for
    that particular branch network)

    You may even have bigger issues in that it could be a switching path
    problem. Your EIGRP packets are process switched and not cef/fast
    switched. So things that work when you fast/cef switch may not work
    when you process switch and vice versa.


    --

    hsb

    "Somehow I imagined this experience would be more rewarding" Calvin
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    Hansang Bae, Dec 19, 2004
    #2
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  3. Andrey Tarasov

    Guest

    See if "ip tcp path-mtu-discovery" works for you.
    , Dec 20, 2004
    #3
  4. Hello, Hansang!
    You wrote on Sun, 19 Dec 2004 04:22:29 GMT:

    ??>> I found some strange thing and wonder if somebody seen it
    ??>> before. I have a Cisco 2651 router with 12.2(24) IOS running
    ??>> EIGRP. There is GRE tunnel to remote site. Everything is working
    ??>> except from the fact that EIGRP updates gets fragmented by GRE.
    ??>> Changing "ip mtu" doesn't do any difference except the case when
    ??>> "ip mtu" set to 1500, than it's GRE packet which gets
    ??>> fragmented. I'm not sure if EIGRP tracks "ip mtu" and just
    ??>> miscalculate packet size or ignores it altogether.
    ??>>
    ??>> I have a case opened with Cisco but all I've got so far is two
    ??>> statements - EIGRP pays attention to MTU and EIGRP doesn't do
    ??>> that.
    ??>>
    HB> I haven't seen this yet but I guess we might run into it as we
    HB> are moving to 12.2.24a for quite a few of the branch routers (but
    HB> OSPF for that particular branch network)

    HB> You may even have bigger issues in that it could be a switching
    HB> path problem. Your EIGRP packets are process switched and not
    HB> cef/fast switched. So things that work when you fast/cef switch
    HB> may not work when you process switch and vice versa.

    Update - it looks like 12.3 mainline might OK since I don't see it from the
    other end running 12.3.12. (Not so much help here for you, Hansang, though) So
    far TAC didn't find any existing DDTS and after quick check of 12.2 code they
    saying it might be GRE not reporting ip mtu properly.

    With best regards,
    Andrey.
    Andrey Tarasov, Dec 28, 2004
    #4
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