OSPF DR and BDR election rules

On broadcast and NBMA networks, the DR and BDR selected by OSPF are electoral, lifelong, and hereditary.
Electoral DR and BDR are not designated manually. Instead, they are elected by all the routers on a network segment.
When the DR and BDR are elected on a network segment and are lifelong, each router newly added to the network segment does not replace the existing the DR or BDR even if the router has a higher priority.
If the DR and BDR are hereditary, the BDR replaces the DR when the DR is faulty and other routers compete to be the BDR.
The process of DR/BDR election on a broadcast or an NMBA link is as follows:

1. After interfaces on a device are Up, the device sends Hello packets and enters the Waiting state. The waiting timer value is the same as that of the dead timer. The default value is 40s and cannot be modified.

2. Before the waiting timer is triggered, the Hello packets sent by the device do not contain the DR and BDR fields. After the device enters the Waiting state, if it sends Hello packets containing the DR and BDR fields, it acknowledges the DR and BDR existing on the network and does not trigger an election. The device changes its state from Waiting and starts synchronization with a neighbor.

3. If the DR and BDR already exist on the network, an added device acknowledges the DR and BDR. In such a case, DR/BDR election is not performed even if this device has a largest router ID or top DR priority.

4. When the DR is Down due to faults, the BDR replaces the DR and another device of a priority higher than 0 becomes the new BDR.

5. DR election is performed only when devices with different router IDs or DR priorities start simultaneously. The election rules are as follows: The device with the highest priority functions as the DR, and the device with the second highest priority functions as the BDR. A device with the priority of 0 can only function as the DR Other. If all devices have the same priority, the device with the largest router ID functions as the DR, the device with the second largest router ID functions as the BDR, and the others as DR Others.
For more information about OSPF DR election and BDR election, click 问鼎OSPF(2) .

Scroll to top