Designated router (DR) and backup router (BDR)

In OSPF a router can have a number of different roles to play.

A designated router (DR) is the designated broadcasting router interface for an AS. It looks after all the initial contact and other routing administration traffic. Having only one router do all this greatly reduces the network traffic and collisions.

If something happens and the designated router goes offline, the backup designated router (BDR) takes over. An OSPF FortiGate unit interface can become either a DR or BDR. Both the DR and the BDR cover the same area, and are elected at the same time. The election process doesn’t have many rules, but the exceptions can become complex.

 

Benefits

The OSPF concept of the designated router is a big step above RIP. With all RIP routers doing their own updates all the time, RIP suffers from frequent and sometimes unnecessary updates that can slow down your network. With OSPF, not only do routing changes only happen when a link-state changes instead of any tiny change to the routing table, but the designated router reduces this overhead traffic even more.

However, smaller network topologies may only have a couple routers besides the designated router. This may seem excessive, but it maintains the proper OSPF form and it will still reduce the administration traffic but to a lesser extent than on a large network. Also, your network topology will be ready whenever you choose to expand your network.

 

DR and BDR election

An election chooses the DR and BDR from all the available routers. The election is primarily based on the priority setting of the routers—the highest priority becomes the DR, and the second highest becomes BDR. To resolve any ties, the router with the highest router ID wins. For example 192.168.0.1 would win over 10.1.1.2.

The router priority can vary from 0 to 255, but at 0 a router will never become a DR or BDR. If a router with a higher priority comes on line after the election, it must wait until after the DR and BDR go offline before it would become the DR.

If the original DR goes offline, but then is available when the BDR goes offline later on, the original DR will be promoted back to DR without an election leaving the new BDR as it is.

With your FortiGate unit, to configure the port1 interface to be a potential OSPF designated router or backup designed router called ospf_DR on the network, you need to raise the priority of the router to a very high number such as 250 out of 255. This will ensure the interface has a chance to be a DR, but will not guarantee that it will be one. Give the interface a low numbered IP address—such as 10.1.1.1 instead of 192.168.1.1—to help ensure it becomes a DR, but that is not part of this example. Enter the following command:

config router ospf config ospf-interface edit “ospf_DR”

set priority 250

end end

 

Area

An OSPF area is a smaller part of the larger OSPF AS. Areas are used to limit the link-state updates that are sent out. The flooding used for these updates would overwhelm a large network, so it is divided into these smaller areas for manageability.

Within an area if there are two or more routers that are viable, there will always be a designated router (DR) and a backup DR (BDR). For more on these router roles, see Designated router (DR) and backup router (BDR) on page 380.

Defining a private OSPF area, involves:

• Assigning a 32-bit number to the area that is unique on your network
• Defining the characteristics of one or more OSPF areas
• Creating associations between the OSPF areas that you defined and the local networks to include in the OSPF area
• If required, adjusting the settings of OSPF-enabled interfaces.

IPv6 OSPF area numbers use the same 32-bit number notation as IPv4 OSPF.

If you are using the web-based manager to perform these tasks, follow the procedures summarized below. FortiGate units support the four main types of OSPF area:

• Backbone area
• Stub area
• NSSA
• Regular area