Designated & Backup Designated Router

1. DESIGNATED & BACKUP
DESIGNATED ROUTER

2.  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 multi-access 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.

3.  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.

4.  On LANs, DR and BDR have to be elected. Two rules are
used to elect a DR and BDR:
1. router with the highest OSPF priority will become a DR.
By default, all routers have a priority of 1
2. if there is a tie, a router with the highest router ID wins
the election
 The router with the second highest OSPF priority or router ID
will become a BDR.

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

6.  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).

7. We can verify that R1 and R2 are indeed the DR and BDR of
the segment by typing the show ip ospf neighbors command
on R3:
Router R3 will be in the 2WAY state with every other non-DR or
non-BDR router.
NOTE – you can influence the DR and BDR election process by
manually configuring OSPF priority. This is done by using the ip
ospf priority VALUE command interface command.

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