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OSPF Reference Bandwidth Question

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  • 1.  OSPF Reference Bandwidth Question

    Posted 07-19-2017 12:07

    Hey guys,

     

    Quick question that I would try if I had a lab available to me.

     

    Studying for the JNCIS, and had a question in regards to the reference bandwidth.

     

    In the material it explicitly says that reference bandwidth is calculated by the physical interface's bandwidth.

     

    But what happens when you force a physical interface to advertise at a certain speed that is slower than it's physical capabilities. 

     

    i.e. If you configure a gigabit interface with the:

     

    [edit]
    
    set interface ge-0/0/0 ether-options speed 100m link-mode full-duplex

    command, will OSPF see it as a 100Mb/s link or a 1Gb/s link?

     

    Because of how it's described in the material, I'm going to go for the OSPF still seeing it as a 1Gb/s link. But maybe not 🙂

     

    Thanks in advance,

     

    Purplezorz



  • 2.  RE: OSPF Reference Bandwidth Question
    Best Answer

    Posted 07-19-2017 12:19

    OSPF will calculate based on the interface speed (so your 100MB connection will be calculated based on it being a 100MB connection and not a 1GB connection.)

     

    The other thing you can do is to set the bandwidth statement

    set interfaces ge-0/0/1 unit 0 bandwidth 3m 
    

    (sets the interface to 3Mb/s)

    this will allow you set your speed even lower that the physical connection speed.

     

    https://www.juniper.net/documentation/en_US/junos12.3/topics/usage-guidelines/interfaces-configuring-the-interface-bandwidth.html

     

     



  • 3.  RE: OSPF Reference Bandwidth Question

    Posted 07-19-2017 14:17

    Hello Matt,

     

    Ahh, I see, so it's cleverer than I thought.

     

    Thank you for your reply.

     

    One more thing, I assume that the "metric" parameter used within an OSPF configuration underneath the interface would be preferred over the bandwith parameter. i.e.

     

    [edit]
    
    interfaces {
              ge-0/0/0 {
                        unit 0 {
                                  bandwidth 3m;
                                  }
                        }
              }
    
    protocols {
              ospf {
                      area 0 {
                               interface ge-0/0/0 {
                                                    metric 100;
                                  }
                        }
              }
    }

    *code is an approximation

     

    Kindest Regards,

     

    Purplezorz