4.2 Steps to OSPF Operation
4.2.3 Step 2: Electing the DR and BDR
When routers first come up on a network, they perform the Hello process, as discussed earlier. In addition, they must elect a Designated Router and Backup Designated Router (DR and BDR) to forward network information to all participating OSPF routers. The DR and BDR add value to the network in the following ways:
  • Reduce routing update traffic --- The DR and BDR act as a central point of contact for link-state information exchanged on a given network; therefore, each router must establish an adjacency with the DR/BDR, as shown in the figure. Instead of each router exchanging link-state information with every other router on the segment, each router sends information about itself in a link-state format (known as a Link State Advertisement, or LSA) to the DR and BDR. The DR sends each router's link-state information to all other routers in the network. This flooding process significantly reduces the router-related traffic on a segment.
  • Manage link-state synchronization --- The DR and BDR ensure that the other routers on the network have the same link-state information about the internetwork. In this way, the number of routing errors is reduced.
Lab Activity
  In this lab, you will learn how to configure OSPF timers.

The BDR does not perform any DR functions when the DR is operating. Instead, it receives all information, but allows the DR to perform the forwarding and synchronization tasks. The BDR performs DR tasks only if the DR fails.

To elect a DR and BDR, the routers view each other's priority value during the Hello packet exchange process, as shown in the figure. The priorities are listed as P=x where x is the priority setting. The router with priority 3 is the highest priority number and will become the Designated Router. The router with the next highest priority (priority 2) is the BDR. Routers with a priority of 0 cannot become a Designated Router or a Backup Designated Router. The following conditions are used to determine which routers are elected as DR and BDR:

  • The router with the highest set priority value is the DR.
  • The router with the second highest set priority value is the BDR.
  • The default value for OSPF interface priority is 1. In case of a tie, the router's Router ID is used as a tie breaker. Thus, when two routers' priority values are equal, the router with the highest ID number becomes the DR.
  • If a router with a higher priority value is added to the network, the DR and BDR do not change. The only time a DR or BDR will change is if one of them goes down. If the DR goes down, the BDR takes over as the DR and a new BDR is elected. If the BDR goes down, a new BDR is elected.
  • To determine whether the DR is down, the BDR sets a timer. This is a reliability feature. If the BDR does not hear the DR forwarding link-state advertisements (LSAs) before the timer expires, the BDR assumes the DR is out of service and the BDR becomes the new DR.

In a multiaccess environment, each network segment, as shown in the main Figure, will have its own DR and BDR. Therefore, a router that is connected to multiple networks can be a DR on one segment and a regular router on another segment. How neighbors are perceived in other network topologies is discussed next.