11.3.3 Broadcast networks and designated routers
On a broadcast network, OSPFv3 selects a designated router and backup designated router to manage the connections between the network at the vertex of the topology and the routers directly connected to the network. To limit disruption to routing behavior, the backup designated router takes over the role of the designated router immediately if the designated router fails.
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(1) Selecting Designated Routers and Backup Designated Routers
Each router advertises its priority to become a designated router on an interface in its hello packets.
If there is no designated router or backup designated router on an interface, OSPFv3 selects the router with the highest priority as the designated router. If the interface has a designated router but lacks a backup designated router, OSPFv3 selects the router with the next highest priority as the backup designated router. The designated router and backup designated router do not give up their roles if a router comes online with a higher priority.
When an interface of a given router has a priority of 0, that router will never be selected as the designated router or backup designated router in an area where the interface is connected.
When multiple routers exist in a broadcast network that is used to forward traffic, an interface of at least one router that connects to the network must have a priority of 1 or higher.