Operation Manual – Routing ProtocolH3C S7500 Series Ethernet Switches Chapter 4 OSPF Configuration4-6DR BDRDRother DRotherDRotherFigure 4-2 DR and BDRIV. DR/BDR electionInstead of being manually configured, DR and BDR are elected by all the routers onthe current network segment. The priority of a router interface determines thequalification of the interface in DR/BDR election. All the routers with DR prioritiesgreater than 0 on the current network segment are eligible "candidates".Hello packets serve as the "votes" in the election. Each router writes the DR it selectsinto the Hello packet and sends the packet to each router running OSPF on thenetwork segment. If two routers on the same network segment declare themselves tobe the DR, the one with the highest DR priority will be preferred. If their priorities arethe same, the one with greater router ID will be preferred. A router whose DR priorityis 0 can neither be elected as the DR nor be elected as the BDR.Note the following points:z DR election is required for broadcast or NBMA interfaces but is not required forP2P or P2MP interfaces.z DR is based on the router interfaces in a certain segment. A router may be a DRon one interface and a BDR or DR Other on another interface.z If a new router is added after DR and BDR election, the router does not becomethe DR immediately even if it has the highest DR priority.z The DR on a network segment is not necessarily the router with the highestpriority. Likewise, the BDR is not necessarily the router with the second-highestpriority.4.1.5 OSPF PacketsOSPF uses five types of packets:z Hello packetHello packets are most commonly used OSPF packets, which are periodically sent bya router to its neighbors. A Hello packet contains the values of some timers, the DR,the BDR and the known peers.z DD packet
