Operation Manual – OSPFH3C S9500 Series Routing Switches Chapter 1 OSPF Configuration1-25Perform the following configuration in OSPF view to configure/remove the NBMAneighbors for OSPF:To do... Use the command...Configure the NBMA neighbors for OSPF peer ip-address [ dr-prioritydr-priority-number ]Remove the configured NBMA neighbors undo peer ip-addressBy default, the preference for NBMA neighbor is 1.1.3.13 Setting the Interface Priority for DR ElectionOn a broadcast or NBMA network, a designated router (DR) and a backup designatedrouter (BDR) must be elected.The priority of a router interface determines the qualification of the interface in DRelection. The router with the priority of 0 cannot be elected as the DR or BDR.DR is not designated manually. Instead, it is elected by all the routers on the segment.Routers with the priorities larger than 0 in the network are eligible “candidates”. Votesare hello packets. Each router writes the expected DR in the packet and sends it to allthe other routers on the segment. If two routers attached to the same segmentconcurrently declare themselves to be the DR, choose the one with higher priority. If thepriorities are the same, choose the one with greater router ID. If the priority of a router is0, it will not be elected as DR or BDR.If DR fails due to some faults, the routers on the network must elect a new DR andsynchronize with the new DR. The process will take a relatively long time, during which,the route calculation is incorrect. In order to speed up this process, OSPF puts forwardthe concept of BDR. In fact, BDR is a backup for DR. DR and BDR are elected in themeantime. The adjacencies are also established between the BDR and all the routerson the segment, and routing information is also exchanged between them. When theDR fails, the BDR will become the DR instantly. Since no re-election is needed and theadjacencies have already been established, the process is very short. But in this case,a new BDR should be elected. Although it will also take a quite long period of time, it willnot exert any influence upon the route calculation.Note the following:z The DR on the network is not necessarily the router with the highest priority.Likewise, the BDR is not necessarily the router with the second highest priority. If anew router is added after DR and BDR election, it is impossible for the router tobecome the DR even if it has the highest priority.z DR is based on the router interface in a certain segment. Maybe a router is a DRon one interface, but can be a BDR or DROther on another interface.
