OSPF - what triggers a DR-BDR election
In an OSPF environment, under normal circumstances, DR/BDR elections should not occur frequently. After the initial election process, the DR and BDR remain in their roles until a trigger occurs.
The DR/BDR election process is triggered by the following events:
- Initial interface up: When OSPF routers are powered up initially, and their interfaces come up, and they create new adjacencies on a network, it starts the DR/BDR election process.
- Interface Down: If an interface of the current DR or BDR goes down, a new DR/BDR election is initiated.
- Restarting OSPF Process: If the OSPF process on a router is restarted, a new DR/BDR election will be triggered on the network segments that router is connected to.
- Network Instability or Outages: Any network instability that causes OSPF adjacencies to reset will trigger a DR/BDR election.
It's important to note, however, that OSPF DR/BDR elections are preemptive. Once a DR and BDR have been elected, they will remain in these roles until they go down or their OSPF processes are restarted. Even if a new router with a higher Router ID or higher priority is added to the network, it won't take over the DR or BDR role unless the current DR or BDR goes down. This is done to maintain stability in the OSPF network.
Links:
https://forum.networklessons.com/t/ospf-dr-bdr-election-explained/880/159?u=lagapides
https://networklessons.com/ospf/ospf-drbdr-election-explained