Designated & Backup Designated Router

Based on the network types, OSPF router can be elect one router to Designated Rutter (DR) and one router to Backup Designated Router (BDR). Let’s take examples, on multi access broadcasts network (such as LANs) router defaults to the select DR and BDR. DR and BDR served as the central point for exchanging OSPF routings information’s. Each non-DR or non-BDR routers will be exchange routings information’s only with DR and BDR, instead of exchanging updates with every router on network segments. DR will distribute topology information to be every other router inside same area, which greatly reduce OSPF traffic.

To the send routings information to DR or BDR. The multicast address of 224.0.0.6 is used. DR send routing updates to multicast address of 224.0.0.5. If DR are fails, BDR takes over its role of redistributing routings information’s.

Every router on segment will be establish full neighbour relationships with DR and BDR. Non-DR and non-BDR routers will be establish a two-way neighbours relationships between themselves. 

On LAN’s, DR and BDR have to be selected. Two rules are used to be select DR and BDR:

1.    Router with highest OSPF priority will be become a DR. By default, all routers have priority of 1.

2.    If there is a tie, a router with highest routers ID wins the elections. The router with seconds highest OSPF priority or routers ID will be become a BDR.

To the better understand concept, consider the following are example.

All router depicted above in the same area (area 0). All router is running OSPF Routers R1 and R2 have been selected as DR and BDR. because they have highest and second highest router ID (100.0.0.0 and 90.0.0.0 respectively). for, example, R3’s directly is connected subnet fails, R3 informs R1 & R2 (the DR and BDR for the segment) of network change (step 1). R1 then informs all other non-DR & non-BDR routers of change in topology (step 2).

We can verify that R1 and R2 indeed the DR and BDR of segment by typing the show ip ospf neighbour command on R3:

R3#show ip ospf neighbor 

Neighbor's ID     Pri   State           Dead Time   Address         Interfaces

60.0.0.0          1   FULL/DROTHER    00:00:33    10.0.0.5        FastEthernet0/0

100.0.0.0         1   FULL/DR         00:00:33    10.0.0.1        FastEthernet0/0

70.0.0.0          1   FULL/DROTHER    00:00:33    10.0.0.4        FastEthernet0/0

90.0.0.0          1   FULL/BDR        00:00:33    10.0.0.2        FastEthernet0/0