8 Replies Latest reply: Jul 22, 2011 10:34 PM by Brian McGahan - 4 x CCIE, CCDE RSS

    ospf DR

    Scott_O'Brien
    Scott_O'Brien Jul 22, 2011 8:36 PM

      so with the ospf election as there is no preempt you have to configure the router that you want to be the DR first? unless you restart the ospf processes then the next router that was configured becomes the DR regardless if there is a router with a better priority.

       

      so say the DR router died and you replace it and set it up so it has the best priority you will have to go one each router and reload the ospf processes? or set all the routers to have a priority of 0?

       

      does this seem very silly to any one else or am i missing somthing?

       

      thanks

       

      EDIT: just changed all my routers to have a priority of 0 and they all become drothers so say your main router does die and your network has all your routers to have priority of 0's as this is the easyest way to get a router to become a dr then your network has no dr or bdr....

         
        • 1. Re: ospf DR
          Martin
          Martin Jul 22, 2011 8:51 PM (in response to Scott_O'Brien)

          DR and BDR must have connections to all other routers, right ?

          so, if someone accidentally turn on router with very high loopback IP, re-election happens right away, you will have a BIG mess on your hands   

          • 2. Re: ospf DR
            Scott_O'Brien
            Scott_O'Brien Jul 22, 2011 8:55 PM (in response to Martin)

            but as there is no pre-empt then even if that router gets turned on unless the current DR gets reloaded or the neighbor relashioships get reset then it would have no effect, and even then if there was a router that was up after befoure the high loop back router on after the DR router then that router would take affect regardless of the priority.

            • 3. Re: ospf DR
              Brian McGahan - 4 x CCIE, CCDE
              Brian McGahan - 4 x CCIE, CCDE Jul 22, 2011 9:18 PM (in response to Scott_O'Brien)

              In reality as long as all the routers on the segment have full layer 2 reachability to each other it doesn't matter who the DR or BDR is.  This design only makes a difference if you have emulated broadcast networks over point-to-point layer 2 technologies such as Frame-Relay or ATM partial-mesh, DMVPN, or VPLS.

               

              In general there is no preemption of the DR in OSPF, but there are corner case designs that can occur which results in preemption, and could potentially result in a loss of connectivity.

               

              The normal behavior should be that the election occurs for the DR and BDR, then if the DR goes down the BDR takes its place, and a re-election occurs for the BDR.

               

              Here is an example case of where this design can break down and cause failures in the topology: http://ieoc.com/forums/p/16073/133783.aspx#133783

              • 4. Re: ospf DR
                Scott_O'Brien
                Scott_O'Brien Jul 22, 2011 9:23 PM (in response to Brian McGahan - 4 x CCIE, CCDE)

                thanks but how do you make it so that only the DR and BDR are part of an election? the only way i can think of is if you set all routers to a priority of 0 except the DR and BDR then if the DR goes down the BDR takes over and when the DR comes back online set the BDR'S priority back to 0 then once the DR is elected again re-set the BDR'S priority back.

                • 5. Re: ospf DR
                  Martin
                  Martin Jul 22, 2011 9:38 PM (in response to Scott_O'Brien)

                  default priority is 1

                  also, do u know about clearing ospf  instead of re-booting routers ?

                  • 6. Re: ospf DR
                    Scott_O'Brien
                    Scott_O'Brien Jul 22, 2011 9:39 PM (in response to Martin)

                    yea clear ip ospf processes, i just think its an odd way and cisco could have done it better

                    • 7. Re: ospf DR
                      Martin
                      Martin Jul 22, 2011 10:09 PM (in response to Scott_O'Brien)

                      have you read this tread ? if not read it

                      https://learningnetwork.cisco.com/message/76448#76448

                      • 8. Re: ospf DR
                        Brian McGahan - 4 x CCIE, CCDE
                        Brian McGahan - 4 x CCIE, CCDE Jul 22, 2011 10:34 PM (in response to Scott_O'Brien)

                        Scott_O'Brien wrote:

                         

                        thanks but how do you make it so that only the DR and BDR are part of an election? the only way i can think of is if you set all routers to a priority of 0 except the DR and BDR then if the DR goes down the BDR takes over and when the DR comes back online set the BDR'S priority back to 0 then once the DR is elected again re-set the BDR'S priority back.

                         

                        If the priority is zero, the router will not participate in the DR/BDR election.  You can technically do what you proposed to influence the election, but you wouldn't want to.  Every time DR is re-elected is causes a reconvergence event, during which packets are dropped until the network can reconverge.  Like I said, in a real design it doesn't matter who the DR or BDR are unless you have some complex non-broadcast or emulated broadcast topology.