4 Replies Latest reply: Mar 30, 2012 2:00 PM by Refrusloi RSS

    DR(OSPF) & Priority?


      I am having a little bit of trouble with the concept of DR. It was explained very briefly in my book, but it seems important.


      I have a feeling I am mixing up Priority from STP with Priority from OSPF


      Priority in STP for Root Switch/Bridge:

      Lower the priority the better (+ mac address)

      No Loopback involved?


      Priority in OSPF for DR:

      Higher the priority the better??

      0 Priority takes it out of the game for DR?

      High Loopback or IP makes it DR?

        • 1. Re: DR(OSPF) & Priority?

          looks OK to me;

          the highest IP on interface (UP UP) or the highest loopback IP if configured.

          • 2. Re: DR(OSPF) & Priority?

            You have it correct, with one caveat for the DR


            The DR election is based on interface priority first.


            Priority is configured on the interface in the range of 0 to 255, with a higher number being better and a zero basically causing the router to not participate in the election on that interface. The default interface priority is 1.


            If the interface priority is the same for all competing routers, then the highest router ID wins.


            The router id is selected one of three ways

            1) via the router-id command as a part of OSPF configuration

            2) the highest addressed loopback interface, if the router-id command is not configured

            3) the highest configured interface address, if there are no loopback interfaces configured.


            The router id is chosen by ospf at the point in time where you configure your first network statement, so if you are going to use the router-id command, it should be the first command you enter after going into router configuration mode for OSPF.


            If you leave the router-id command out and/or want to change the router id, you have to set the configuration and then restart the OSPF process, either by saving your configuration and rebooting the router (ouch) or issuing the clear ip ospf process command.


            The loopback or physical interfaces do not have to be configured for OSPF to be considered for the router id; that is there does not have to be a network statement that includes that particular interface's network or subnet.





            • 3. Re: DR(OSPF) & Priority?

              if yoiu have not apply RID manualy using loopback, then the interface with highest ip address will become RID.


              & its always better to configure RID manually.


              (~)# router ospf 1234

              (~)# router-id


              hope it helps....!!!

              • 4. Re: DR(OSPF) & Priority?

                Thanks guys. That really helped, I got it now!!