DR and BDR Concepts in OSPF

For previous post about Neighbourship formation....Click here neighbour-formation-stages-in-ospf

àIn a network all the router will sends route updates to all other routers, so that the burden is very high on the network due to the routing updates traffic. 

      To solve this DR and BDR is introduced. So that all the routers will send route updates to these DR and BDR only

Ø  To reduce the FULL Neighbourships.

Ø  To reduce the CPU burden on devices.

Ø  To reduce the Bandwidth utilization.

So that all routers will make FULL Neighbourship with DR, while the other router will make up to 2-Way state neghbourship only.

Designated & Backup Designated Router (DR/BDR):

Based on the network type, OSPF router can elect one router to be a Designated Router (DR) and one router to be a Backup Designated Router (BDR). For example, on multiaccess broadcast networks (such as LANs) routers defaults to elect a DR and BDR. DR and BDR serve as the central point for exchanging OSPF routing information. Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment. DR will then distribute topology information to every other router inside the same area. This greatly reduces OSPF traffic.

To send routing information to a DR or BDR the multicast address of 224.0.0.6 is used. DR sends routing updates to the multicast address of 224.0.0.5. If DR fails, BDR takes over its role of redistributing routing information.

Every router on a network segment establish a full neighbor relationship with the DR and BDR. Non-DR and non-BDR routers establish a two way neighbor relationship between themselves.

NOTE – on point-to-point links a DR and BDR are not elected since only two routers are directly connected.

 Two rules are used to elect a DR and BDR:

ü  router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1

Ø  We can change the Priority interface mode.

ü  if there is a tie, a router with the highest router ID wins the election

ü  The router with the second highest OSPF priority or router ID will become a BDR.

To better understand the concept, consider the following example.

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

Note: in OSPF we can change the selected path by:

Ø  Bandwidth

Ø  Cost

Ø  Reference Bandwidth

#Conf t

#Int s0/0

#Bandwidth 1000 (Kbps)

#Ip ospf cost 5

#Router ospf 10

#Auto-cost referance bandwidth 1000

In OSPF Areas are of two types:

1. Backbone area – Area 0 (It should be Zero only)

2. Regular area – Area 1, 2…

Note: All regular areas are must be adjacent to backbone area

Route types in OSPF:

1. Intra- area route à O

2. Inter – area route àOIA

3. External route àOE1 – Cost will varied

                                OE2 – Cost will same

4. External route via NSSA (Not-So-Stubby  Area)

            Whenever the OSPF is connected to the different protocol like EIGRP we have to redstribute the EIGRP to OSPF. Those routes are represented by OE2

            External routes via NSSA area as ON2, the other router as OE2