v22 and nvram

Discussion in 'Tomato Firmware' started by pfoomer, Nov 30, 2008.

  1. pfoomer

    pfoomer LI Guru Member


    upgraded to Victek's v22 ND, cleared nvram, re entered all settings by hand.

    Changed MTU to 1500 by gui, but its back to 1492 after commit & reboot.

    telnet in, changed to 1500, did a commit & reboot, back to 1492.

    Avoid performing a NVRAM option unchecked.

    Worked ok in v21, so have I goofed?
  2. spicoli

    spicoli LI Guru Member

    I recently changed my MTUs just fine, so it could be something on your end. Also, I typically have the "Avoid..." command checked to prevent multiple writes to nvram over some "petty" stuff (changing QoS rules for one). Call me paranoid, but don't wanna overdo it on the nvram. I only commit after I have everything setup, so basically its only one write, instead of every time I hit the save button. If I'm wrong or this shouldn't be done, lemme know. :p
  3. fyellin

    fyellin LI Guru Member

    There's code in rc/wan.c that doesn't let the mtu go above 1492 if you're running PPPOE and over 1460 if you're running PPTP or L2TP. Other modes have a max of 1500.

    I don't know how long this code has been there.
  4. pfoomer

    pfoomer LI Guru Member


    Thanks for that. I was sure I set it to 1500, but on reflection maybe it did not survive a reboot, any way, the purpose was to optimise the MTU.

    Pinging my ISP gets a max of 1464 + 28 = 1492!
    For efficency is it now better to set all the LAN clients to 1492, the default is 1500?

    My query really is is there an issue for the router re different LAN/WAN packet sizes?

  5. mstombs

    mstombs Network Guru Member

    There is usually a maximum in the internet path between your PC and the Internet of 1500, PPPOE adds an overhead of 8 bytes which means the maximum packet that can be set is 1492. Your PCs on the LAN should auto detect the maximum - otherwise certain websites will not work correctly. Its possible the router can 'fragment' oversize packets, but this is very inefficient and probably breaks some protocols.

    I have to use an even lower MTU on my PC as a CISCO VPN client also adds overhead when working from home.

    There's some information on the net I can't locate right now that gives theoretical optimum MTU sizes for PPPoA and PPPoE, its not the maximum because of the breakdown into ATM frames

    Here's some old links, I'm sure here were later revisions, maybe on the Aus whirlpool forums

    http://www.mynet watchman.com/kb/adsl/pppoemtu.htm

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