If the network loses the DR, the BDR immediately becomes the new DR on the broadcast segment. After OSPF elects the new DR, all routers perform an SPF run and issue new LSAs for the segment. The new DR generates a new network LSA for the segment and every router on the segment must refresh the router LSA.
Each router performs the SPF run as soon as it detects a new DR. Depending on the speed of the router, the router can perform the SPF run before it receives the new LSAs for the segments, which requires a second SPF run to update and continue routing across the segment. The OSPF hold-down timer does not permit two consecutive SPF runs within the value of the timer. This limitation can lead to traffic interruption of up to 10 seconds.
In a classical OSPF routed design, this situation never causes a problem because OSPF runs over multiple segments so even if a segment is not usable, routes are recalculated over alternative segments. Typical Routed Split MultiLink Trunnking (RSMLT) designs only deploy a single OSPF routed vlan, which constitutes a single segment.
You can use RSMLT in a configuration with dual core VLANs to minimize traffic interruption when the network loses the DR. This configuration creates a second OSPF core VLAN, forcing different nodes to become the DR for each VLAN. Each OSPF core VLAN has a DR (priority of 100) and no BDRs. This configuration does not require a BDR because the two VLANs provide backup for each other from a routing perspective. See the following figure for a network example.
