Couldn't increase MxU to 1500 and PPPoE - PPP

This is a discussion on Couldn't increase MxU to 1500 and PPPoE - PPP ; When PPPoE is used, every tutorial say that MTU and MRU should be set to 1492. But when starting a session, there is multiple warnings when the session is started and halted This log is from ppp-2.4.2 and rp-pppoe.so plugin ...

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Thread: Couldn't increase MxU to 1500 and PPPoE

  1. Couldn't increase MxU to 1500 and PPPoE

    When PPPoE is used, every tutorial say that MTU and MRU should be set to
    1492.

    But when starting a session, there is multiple warnings when the session is
    started and halted
    This log is from ppp-2.4.2 and rp-pppoe.so plugin
    Aug 28 15:08:44 ipcop pppd[4622]: Couldn't increase MTU to 1500
    Aug 28 15:08:44 ipcop pppd[4622]: Couldn't increase MRU to 1500
    Aug 28 15:08:47 ipcop pppd[4622]: Couldn't increase MRU to 1500
    Aug 28 15:08:50 ipcop pppd[4622]: Couldn't increase MTU to 1500
    Aug 28 15:08:50 ipcop pppd[4622]: Couldn't increase MRU to 1500
    Aug 28 15:08:50 ipcop pppd[4622]: Couldn't increase MTU to 1500
    Aug 28 15:08:50 ipcop pppd[4622]: Couldn't increase MRU to 1500
    Aug 28 17:29:25 ipcop pppd[4622]: Couldn't increase MTU to 1500
    Aug 28 17:29:25 ipcop pppd[4622]: Couldn't increase MRU to 1500

    Is ppp-2.4.2 wrong reporting the warning?
    Or should we not set MTU and MRU to 1492 and hope the ISP have correctly set
    the BAS, so lcp negociation will correctly set MTU and MRU to 1492?



  2. Re: Couldn't increase MxU to 1500 and PPPoE

    "Gilles Espinasse" writes:
    > When PPPoE is used, every tutorial say that MTU and MRU should be set to
    > 1492.


    Yes. PPPoE cannot be used with an MTU or MRU greater than 1492, as
    the Ethernet maximum is 1500 octets, and the PPPoE headers take up 8
    octets.

    > But when starting a session, there is multiple warnings when the session is
    > started and halted
    > This log is from ppp-2.4.2 and rp-pppoe.so plugin
    > Aug 28 15:08:44 ipcop pppd[4622]: Couldn't increase MTU to 1500


    At a guess, the peer is configured to reject MTU and MRU options, and
    the local implementation doesn't deal with that event cleanly.

    Debug logs (showing the actual negotiation) should be able to settle
    the matter.

    > Is ppp-2.4.2 wrong reporting the warning?
    > Or should we not set MTU and MRU to 1492 and hope the ISP have correctly set
    > the BAS, so lcp negociation will correctly set MTU and MRU to 1492?


    "BAS"?

    I would leave MTU and MRU alone and let PPP do its thing. Don't
    configure what doesn't *need* to be configured.

    --
    James Carlson, IP Systems Group
    Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677

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