A Query about PBR and MTU size.

Discussion in 'Cisco' started by Peter, Mar 16, 2007.

  1. Peter

    Peter Guest

    Hi Folks,

    Can PBR be used to modify the outgoing MTU size?

    I have a scenario with a Cisco 827 using PPPoA and an MTU of 1500,
    where one particular destination site uses PPPoE (and an MTU of 1492)
    and their ISP seems to be blocking Path MTU Discovery from working. If
    I manualy adjust my MTU down I can get this working, however this has
    to be doen for ALL sites, and I just want it to be done for the one
    site. Can I use PBR to do this for just the ONE target Network? If so,
    would someone care to post an example?

    Is there another option for resolvnig this problem?

    Thanks..................pk.

    --
    Peter from Auckland.
     
    Peter, Mar 16, 2007
    #1
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  2. Peter

    Guest

    On 16 Mar, 01:53, "Peter" <> wrote:
    > Hi Folks,
    >
    > Can PBR be used to modify the outgoing MTU size?
    >
    > I have a scenario with a Cisco 827 using PPPoA and an MTU of 1500,
    > where one particular destination site uses PPPoE (and an MTU of 1492)
    > and their ISP seems to be blocking Path MTU Discovery from working. If
    > I manualy adjust my MTU down I can get this working, however this has
    > to be doen for ALL sites, and I just want it to be done for the one
    > site. Can I use PBR to do this for just the ONE target Network? If so,
    > would someone care to post an example?
    >
    > Is there another option for resolvnig this problem?



    I am pretty sure that you are a regular here and will
    know already that the increase in overhead by
    moving from 1500 to 1492 will be so small that
    the effect will be hard to measure. Still I have an idea.

    This may or may not work.

    Set up a new loopback for this
    with an mtu of 1492. Make up an IP address
    for it. Doesn't matter at all what it is as long as there is
    no conflict.
    Make it a NAT inside if necessary.


    Use PBR to send all inbound traffic from your LAN
    via this loopback /if/ it is heading for the PPPPoE site.

    I have seen this type of thing used and work but
    not necesarily in this context.

    I very much like tcp mss-adjust, you may be
    able to use that too.

    Please feed back the outcome.

    Good luck, I wouldn't bother tho':)
     
    , Mar 16, 2007
    #2
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  3. Peter

    Peter Guest

    Greetings,

    > I am pretty sure that you are a regular here


    Yep, been following this group for many years now.........;-)

    > and will know already that the increase in overhead by
    > moving from 1500 to 1492 will be so small that
    > the effect will be hard to measure.


    I agree, performance wise the difference in MTU size is trivial,
    however when some brain dead ISP decides to turn off PMTUD the result
    can be (ultimately) fatal for such things as FTP transfers for files
    over around 80KB. On the odd times I needed it, I set all traffic to
    an MTU of 1492 and the transfers completed fine.

    > Still I have an idea.
    >
    > This may or may not work.
    >
    > Set up a new loopback for this
    > with an mtu of 1492. Make up an IP address
    > for it. Doesn't matter at all what it is as long as there is
    > no conflict.
    > Make it a NAT inside if necessary.
    >
    >
    > Use PBR to send all inbound traffic from your LAN
    > via this loopback /if/ it is heading for the PPPPoE site.
    >
    > I have seen this type of thing used and work but
    > not necesarily in this context.
    >
    > I very much like tcp mss-adjust, you may be
    > able to use that too.
    >
    > Please feed back the outcome.
    >


    Thanks, I will see if I can get it to work.

    > Good luck, I wouldn't bother tho':)


    ..........;-)

    Cheers................pk.


    --
    Peter from Auckland.
     
    Peter, Mar 17, 2007
    #3
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