OSPF BDR to DR Transition Time

In an OSPF network where a DR/BDR election takes place, in the event that the current DR fails, how long does it take for the BDR to take over the role of the DR?

The BDR maintains the same OSPF database and adjacencies as the DR within a network segment so that it can take over without the need for OSPF reconvergence. However, unlike the DR, the BDR does not generate network Link-State Advertisements (LSAs) during regular operation.

The transition of the BDR to DR status occurs when the current DR fails, such as through power loss, link failure, or router shutdown. This transition is dependent on the OSPF hello timer and dead interval settings.

The DR failure is detected when OSPF routers on the segment stop receiving Hello packets from the DR, triggering a reaction only after the dead interval elapses, typically set to 40 seconds by default. Upon expiry of the dead timer, the BDR assumes the DR role immediately and begins broadcasting Type 2 LSAs to inform all routers of the new topology. This process ensures the continuation of traffic routing using established routes.

Links

https://networklessons.com/ospf/ospf-drbdr-election-explained/ https://www.ietf.org/rfc/rfc2328.txt