Routing

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Ask questions and share experiences about ACX Series, CTP Series, MX Series, PTX Series, SSR Series, JRR Series, and all things routing, including portfolios and protocols.

  • 1.  Clear IP DF-Bit without tunnel

    Posted 10-02-2012 06:46

    I need to run MPLS over an Ethernet link on an MX-240 which only supports the standard 1518 byte MTU.

     

    How can I clear the IP DF-bit on packets transiting this link?

    I have found lots of methods for clearing the IP DF-bit on IPSEC/GRE tunnels, but nothing that includes a generic method (similar to the way Cisco does this on any interface with a route-map).

    We have an MS-DPC card if that helps matters.

     

    This link provides a nice method for modifying TCP MSS, but I do not see any options for clearing the DF-bit here other than on tunnel interfaces:

    http://kb.juniper.net/InfoCenter/index?page=content&id=KB24352&cat=TUNNELING_PROTOCOLS&actp=LIST&showDraft=false

     

    Thanks!



  • 2.  RE: Clear IP DF-Bit without tunnel
    Best Answer

    Posted 10-02-2012 08:22

    Hello,

    Currently there is no way to do clear DF bit on transit user packets on MX platform, be it with MSDPC or without. 

    To solve your particular problem, you can run MPLS over GRE across said link, and clear DF bit on GRE packets after encapsulation.

    GRE reassembly must be done by MSDPC.

    Do not foget to enable PMTUD on GRE tunnel as well, and use MSDPC to adjust TCP MSS.

    TCP MSS adjust must be done before GRE encapsulation and/or after decapsulation.

    HTH

    Rgds

    Alex