DR/BDR Election Process


DR/BDR elections do not occur in point-to-point networks. Therefore, in a standard three-router topology, R1, R2, and R3 do not need to elect a DR and BDR, because the links between these routers are not multiaccess networks.
DR/BDR Election
How do the DR and BDR get elected? The following criteria are applied:
1. DR: Router with the highest OSPF interface priority.
2. BDR: Router with the second highest OSPF interface priority.
3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.
In this example, the default OSPF interface priority is 1. As a result, based on the selection criteria listed above, the OSPF router ID is used to elect the DR and BDR. As you can see, RouterC becomes the DR and RouterB, with the second highest router ID, becomes the BDR. Because RouterA is not elected as either the DR or BDR, it becomes the DROther.
DROthers only form FULL adjacencies with the DR and BDR, but will still form a neighbor adjacency with any DROthers that join the network. This means that all DROther routers in the multiaccess network still receive Hello packets from all other DROther routers. In this way, they are aware of all routers in the network. When two DROther routers form a neighbor adjacency, the neighbor state is displayed as 2WAY. The different neighbor states are discussed in CCNP.
The command output in the figure displays the neighbor adjacency of each router on the multiaccess network. Notice for RouterA that it shows that the DR is RouterC with the router ID of 192.168.31.33 and that the BDR is RouterB with the router ID of 192.168.31.22.
Because RouterA shows both its neighbors as the DR and BDR, RouterA is a DROther. This can be verified using the show ip ospf interface fastethernet 0/0 command on RouterA, as shown in the figure. This command will show the DR, BDR, or DROTHER state of this router, along with the router ID of the DR and BDR on this multiaccess network.
Timing of DR/BDR Election
The DR and BDR election process takes place as soon as the first router with an OSPF enabled interface is active on the multiaccess network. This can happen when the routers are powered-on or when the OSPF network command for that interface is configured. The election process only takes a few seconds. If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router ID will become the DR. This could be a lower-end router that took less time to boot.
The role of the DR and BDR will be discussed in more detail in a later section. For now, it is important to know that when the DR is elected, it remains the DR until one of the following conditions occurs:
The DR fails.
The OSPF process on the DR fails.
The multiaccess interface on the DR fails.
In the figure, a red X indicates one or more of these failures.
If the DR fails, the BDR assumes the role of DR and an election is held to choose a new BDR. In the figure, RouterC fails and the former BDR, RouterB, becomes DR. The only other router available to be BDR is RouterA.
If a new router enters the network after the DR and BDR have been elected, it will not become the DR or the BDR even if it has a higher OSPF interface priority or router ID than the current DR or BDR. The current DR and BDR must both fail before the new router can be elected DR or BDR. In the figure, RouterD joins the network. Even though its router ID, 192.168.31.44, is higher than the current DR and BDR routers, RouterD assumes the role of a DROther.
A previous DR does not regain DR status if it returns to the network. In the figure, RouterC has finished a reboot and becomes a DROther even though its router ID, 192.168.31.33, is higher than the current DR and BDR.
Only after both the DR and the BDR fail will the DR and BDR routers change. In the figure, RouterB fails. Because RouterD is the current BDR, it is promoted to DR. RouterC becomes the BDR.
So, how do you make sure that the routers you want to be DR and BDR win the election? Without further configurations, the solution is to either:
Boot up the DR first, followed by the BDR, and then boot all other routers, or
Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.
However, as you may have already guessed, we can change the OSPF interface priority to better control our DR/BDR elections.

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