Expand all | Collapse all

OSPF Reference Bandwidth Question

Jump to Best Answer
  • 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:


    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,



  • 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.




  • 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.


    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,