Designated routers (DRs)
In an OSPF network having two or more routers, one router is elected to serve as the DR and another router to act as the BDR. All other routers in the area forward their routing information to the DR and BDR, and the DR forwards this information to all of the routers in the network. This minimizes the amount of repetitive information that is forwarded on the network by eliminating the need for each individual router in the area to forward its routing information to all other routers in the network. If the area includes multiple networks, each network elects its own DR and BDR.
In an OSPF network with no DR and no BDR, the neighboring router with the highest priority is elected the DR, and the router with the next highest priority is elected the BDR. If the DR goes off-line, the BDR automatically becomes the DR, and the router with the next highest priority then becomes the new BDR. If multiple HPE routing switches on the same OSPF network are declaring themselves DRs, both priority and router ID are used to select the DR and BDRs.
Priority is configurable by using the vlan
command at the interface level. You can use this parameter to help bias one router as the DR. If two neighbors share the same priority, the router with the highest router ID is designated the DR. The router with the next highest router ID is designated the BDR.
vid
ip ospf priority 0-255
For example, in the following figure, the DR and BDR for 10.10.10.0 network in area 5 are determined as follows:
Router A |
Priority: 0 |
Cannot become a DR or BDR |
Router B |
Priority: 1 |
DR for the 10.10.10.0 network |
Router C |
Priority: 2 |
BDR for the 10.10.10.0 network |
Router D |
Priority: 3 |
Cannot become a DR or BDR |
Router E |
Priority: 4 |
Becomes the new BDR if router B becomes unavailable and router C becomes the new DR |
To learn the router priority on an interface, use the
show ip ospf interface
command and check the Pri setting under OSPF interface configuration.
By default, the router ID is typically the lowest-numbered IP address or the lowest-numbered (user-configured) loopback interface configured on the device.
If multiple networks exist in the same OSPF area, the recommended approach is to ensure that each network uses a different router as its DR. Otherwise, if a router is a DR for more than one network, latency in the router could increase because of the increased traffic load resulting from multiple DR assignments.
When only one router on an OSPF network claims the DR role despite neighboring routers with higher priorities or router IDs, this router remains the DR. This is also true for BDRs.
Interface is in a waiting state and the wait time expires
Interface is in a waiting state and a hello packet is received that addresses the BDR
- Change in the neighbor state occurs, such as:
Neighbor state transitions from 2 or higher
Communication to a neighbor is lost
Neighbor declares itself to be the DR or BDR for the first time