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OSPF internals: how the router measure the link speed ?

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  • OSPF internals: how the router measure the link speed ?

    Hello OSPF gurus,

    I have a question not related to any particular cisco device but more on OSPF itself. You certainly agree that OSPF know the speed of any link in the network in order to compute the shortest path tree.

    I wonder how this speed is known by the routers. Does the router send a packet and measure the round trip time ? Or maybe the router know that a ethernet 100mbit link has a speed of 100mb/s and it doesn't measure anything. So, to sum up, is the link speed a dynamic or static information in OSPF ?

    Thank you
    bibop

  • #2
    Re: OSPF internals: how the router measure the link speed ?

    This might answer your question
    http://www.cisco.com/en/US/tech/tk36...80094e9e.shtml
    Marcel
    Technical Consultant
    Netherlands
    http://www.phetios.com
    http://blog.nessus.nl

    MCITP(EA, SA), MCSA/E 2003:Security, CCNA, SNAF, DCUCI, CCSA/E/E+ (R60), VCP4/5, NCDA, NCIE - SAN, NCIE - BR, EMCPE
    "No matter how secure, there is always the human factor."

    "Enjoy life today, tomorrow may never come."
    "If you're going through hell, keep going. ~Winston Churchill"

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    • #3
      Re: OSPF internals: how the router measure the link speed ?

      The more interesting paragraph was "OSPF Cost"

      It says: "The cost of an interface is inversely proportional to the bandwidth of that interface". But i already knew that.

      The question is "does the router measure the bandwidth by performing a test on the link, or it simply knows the speed from the type of the interface ?"

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      • #4
        Re: OSPF internals: how the router measure the link speed ?

        IIRC he uses the speed configured on the interface.
        Marcel
        Technical Consultant
        Netherlands
        http://www.phetios.com
        http://blog.nessus.nl

        MCITP(EA, SA), MCSA/E 2003:Security, CCNA, SNAF, DCUCI, CCSA/E/E+ (R60), VCP4/5, NCDA, NCIE - SAN, NCIE - BR, EMCPE
        "No matter how secure, there is always the human factor."

        "Enjoy life today, tomorrow may never come."
        "If you're going through hell, keep going. ~Winston Churchill"

        Comment


        • #5
          Re: OSPF internals: how the router measure the link speed ?

          The cost of an interface is inversely proportional to the bandwidth of that interface

          OSPF uses the configured bandwidth on the interface by default.

          100000000 bits /interface bandwidth by default. So if its a 100mb interface the cost is 1 by default. Now this can be an issue if your using gig interaces as the cost will always be 1 by default. This can be changed with the auto cost reference bandwidth command. You can also manipulate costs by assigning a higher/lower bandwidth to the interface.
          CCNA, CCNA-Security, CCNP
          CCIE Security (In Progress)

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