What are DR, BDR, DR other and election rules

DR means designated router. BDR means backup designated router. DR Other indicates a device that is neither a DR or a BDR. The DR advertises link-state advertisements (LSAs) to all the devices in the network.

The DR election rules are as follows:

When going Up, an interface sends Hello messages and enters the waiting state. In the waiting state, a waiting timer is triggered. The waiting timer duration is the same as the dead timer duration. By default, the waiting timer duration is 40 seconds, which cannot be changed.
Before the waiting timer is triggered, sent Hello messages carry no DR or BDR field. In the waiting state, if a received Hello message carries the DR and BDR fields, the DR and BDR are accepted directly without any election triggered, and neighbor state synchronization starts, directly exiting the waiting state.
Assume that a DR and a BDR exist on the network. Any device newly connected to the network will accept the DR and BDR that exist on the network regardless of the router ID of the device.
If the DR fails and goes down, the BDR takes over the role of the DR and the remaining devices whose priority is greater than 0 compete to become the new BDR.
DR election rules are used to elect a DR only when devices with different router IDs or configured with different DR priorities are started at the same time. The election rules are that the device with the highest DR priority is elected as DR and the device with the second highest DR priority as BDR. A device with a DR priority of 0 can be a DR Other only. In the case of the same priority, the device with the greatest router ID is elected as DR, the device the second greatest router ID becomes the BDR, and other devices are DR Others.

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