OSPF DR/BDR

What is DR/BDR ?

DR (Designed Router)/BDR (Backup Designated Router) is a router that is chosen to efficiently synchronize LSDB on a multi-access network such as Ethernet. The DR is an OSPF router representing a multi-access network. And the BDR is the backup of the DR. If the DR goes down, the BDR becomes the next DR. The OSPF router that is neither the DR nor the BDR in a multi-access network is the DROTHER.

This section explains in detail the mechanism of synchronizing LSDB on a multi-access network by DR.

Synchronization of LSDB over Ethernet (multi-access network)

In a multi-access network such as Ethernet, two or more OSPF routers may be connected. Each OSPF router on the multi-access network will then eventually synchronize its LSDB.

Consider a simple multi-access network example. In the following figure, four OSPF routers, R1 to R4, are connected on the Ethernet. Initially, each router has only its own LSA registered in its LSDB. It will eventually synchronize the LSDB of each router so that all LSAs of R1 to R4 are registered in the LSDB of R1 to R4.

LSDB synchronization via DR

In order to achieve this synchronization, OSPF elects a DR on the multi-access network; it establishes an adjacency between the DR and the OSPF routers on the multi-access network. Each router sends its own LSA to the DR. Each router sends its own LSA to the DR, which means that all LSAs will be gathered in the DR.

Then, have the DR send the missing LSAs, and finally synchronize the LSDBs of all OSPF routers on the multi-access network.

DR and neighbor/adjacency

The relationship between R1 to R4 neighbors and the adjacency is as follows.

Only the relationship with R1, which is the DR, is an adjacency. All other routers are just neighbors. Routers in a simple neighbor relationship do not exchange LSAs directly with each other, but they ultimately have the same LSDB.

Related article

Please see the following articles about neighbor and adjacency.

N-Study

OSPF Neighbor and Adjacency | N-Study

https://www.n-study.com/en/ospf-detail/ospf-neighbor-adjacency/

There are two types of relationships between OSPF routers: neighbor and adjacency. This section describes the two relationships between OSPF routers.

See the following article on how Neighbors and Adjacencies look on Ethernet with Cisco routers.

N-Study

How show ip ospf neighbor looks on Ethernet | N-Study

https://www.n-study.com/en/ospf-detail/show-ip-ospf-neighbor-on-ethernet/

Configure OSPF on the four Cisco routers on the Ethernet and make sure that they are only adjacent with DR.

Election of DR/BDR

The election of DR/BDR is done by the following two parameters.

• router priority
• router ID

Router Priority and Router ID are both included in the Hello packet. The router priority is an 8-bit value set for the OSPF interface; in decimal, the value ranges from 0 to 255. The router with the highest router priority value becomes the DR, and the next highest router becomes the BDR. And, Priority “0” means that it will not be DR/BDR.

When the router priority has the same value and DR/BDR cannot be determined by router priority, selection by router ID is performed. The router with the highest router ID becomes the DR, and the router with the next highest router ID becomes the BDR. Since the router ID is unique, the router ID will always determine the DR/BDR in the end.

Related article

For information on configuring Cisco routers to change router priority, see the following article.

N-Study

Configuring and Verifying OSPF Router Priority | N-Study

https://www.n-study.com/en/ospf-detail/ospf-priority-configuration/

This section describes the commands for configuring and verifying the router priority value to determine DR/BDR on Cisco routers.

Switching DR

LSDB synchronization is ensured on a multi-access network with the DR at the center; if the DR is changed, LSDB synchronization cannot be maintained and packets can no longer be routed. So, Once a DR/BDR is determined, it is kept as unchanged as possible. Even if a router with a higher router priority is added later, the existing DR/BDR will not change. Also, when a DR goes down, the BDR becomes the new DR.

In the following figure, R1’s router priority is 5, making it a DR, and R2’s router priority is 2, making it a BDR. Even if we add a new R5 with router priority 10, the DR and BDR will not change.

Then, if R1, the DR, goes down, R2, which was the BDR, becomes the new DR. R5 does not become the DR; the BDR is the router that will always become the next DR when the current DR becomes unavailable. In this case, R5 becomes the BDR.

As described above, the behavior is such that as little DR/BDR change as possible will occur. Therefore, when there are multiple OSPF routers in a multi-access network, the order in which the routers are started should be considered, because if the router that you want to be DR starts late, it may not become DR even if its priority is high.

Example of DR election

So far, we have been thinking about DR/BDR using a single Ethernet network as an example. Let’s consider DR election in a more realistic network topology as follows.

The point-to-point network between R1 and R5 in this figure is a serial connection. Everything else is assumed to be an Ethernet multi-access network. Also, the OSPF router priority is set to 1 for all other parts not shown.

The DR/BDR is elected for each multi-access network. In a point-to-point network, DR/BDR election is not required. In this example, the DR election is as follows.

In 192.168.12.0/24, R2 with high router priority is the DR; in 192.168.134.0/24, R1 with high router priority is the DR. Then, in a multiple-access network, it becomes the adjacency and synchronizes the LSDB with the DR. In a point-to-point network, the DR is not elected, but R1 and R5 are the neighbors i.e., they are the adjacency and synchronize the LSDB. Eventually, the LSDBs of all OSPF routers in R1 to R5 will be synchronized.

Summary