Content

| 6.2 |  | OSPF Operation |
| 6.2.3 |  | Step 2: Elect a DR and a BDR |

Because multiaccess networks can support more than two routers, OSPF elects a DR to be the focal point of all link-state updates and LSAs. The role of the DR is critical, therefore a BDR is elected to "shadow" the DR. In the event that the DR fails, the BDR can smoothly take over.

Like any election, the DR/BDR selection process can be rigged to change the outcome. The "ballots" are Hello packets, which contain the ID and priority fields of the router. The router with the highest priority value among adjacent neighbors wins the election and becomes the DR. The router with the second highest priority is elected the BDR. When the DR and BDR have been elected, they keep their roles until one of them fails, even if additional routers with higher priorities show up on the network. Hello packets inform newcomers of the identity of the existing DR and BDR.

By default, all OSPF routers all have the same priority value of 1. A priority number from 0 to 255 can be assigned on any given OSPF interface. A priority of 0 prevents the router from winning any election on that interface. A priority of 255 ensures at least a tie. The Router ID field is used to break ties. If two routers have the same priority, the router with the highest ID will be selected. The router ID can be manipulated by configuring an address on a loopback interface, although that is not the preferred way to control the DR/BDR election process. The priority value should be used instead because each interface can have its own unique priority value. A router can be easily configured to win an election on one interface, and lose an election on another.

How does the DR election process affect the example network? RTB and RTC are connected by way of PPP on a point-to-point link. Therefore, there is no need for a DR on the network 10.6.0.0/16 because only two routers can exist on this link.

Because 10.4.0.0/16 and 10.5.0.0/16 networks are multiaccess Ethernet networks, they may potentially connect more than two routers. Even if only one router is connected to a multiaccess segment, a DR is still elected. This is because the potential exists for more routers to be added to the network. Therefore, a DR must be elected on both 10.4.0.0/16 and 10.5.0.0/16.

> Note: DRs and BDRs are elected on a per-network basis. An OSPF area can contain more than one IP network. Therefore, each area can, and usually does, have multiple DRs and BDRs.

In the example topology, RTA serves a dual role as both the DR and the BDR. Because it is the only router on the 10.4.0.0/16 network, RTA elects itself as the DR. After all, the 10.4.0.0/16 network is a multiaccess Ethernet network. A DR is elected because multiple routers could potentially be added to this network. RTA is also the runner-up in the election for 10.5.0.0/16 and therefore the BDR for that network. Despite claiming equal priority value with RTA, RTB is elected as DR for 10.5.0.0/16 by virtue of the tiebreaker. The tiebreaker is having a higher router ID of 10.5.0.2 versus 10.5.0.1.

With elections complete and bidirectional communication established, routers are ready to share routing information with adjacent routers and build their link-state databases. This process is discussed in the next section.

| Interactive Media Activity Checkbox: DR and BDR Election Upon completion of this activity, the student will be able to identify the process of DR and BDR election.