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Different types of networks present
OSPF with a unique challenge to manage. A network could be point–to–point or a
multiple access network providing a shared medium for multiple routers to
communicate. In a multiple access network, if each router floods the network
with LSAs, the same information about a link state will be forwarded from
multiple sources, leading to a large amount of router CPU load and bandwidth
consumption. In a multi–access network, OSPF uses a single router called
designated router (DR) to control how LSAs are flooded. The purpose of using
the DR is to minimize the number of adjacencies formed so that all topology
tables on routers can be synchronized.

A backup designated router (BDR) is a
hot standby router for the DR in the same network type. The BDR receives LSA
packets and routing updates from OSPF adjacent routers but does not flood the
LSA updates. The BDR only works if the DR fails. Each router in a multiple
access network establishes adjacency with the DR and the BDR.

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Election of the DR and the BDR is won based on
information in the Hello packet. What happens is that when OSPF sends a Hello
packet via an interface to other routers, it will set the priority of the DR
and the BDR fields if it knows which routers are the DR and the BDR. If no
routers declare themselves as the DR or the BDR, the routers then follow an
election procedure which is solely dependent on which router interfaces have
the highest priority. A router whose interface has the highest priority is
elected as the DR. The highest router priority by default is 1. This means that
if the value of a router interface is changed to 0, it prevents that router
from being elected as the DR or the BDR. Also, if the routers have the same
router priority, router ID is used as the tiebreaker. Basically, what happens
is that whenever there is a change in a link status, instead of flooding each
and every path with LSA packets, OSPFv3 only sends the updates to the DR which
then floods all the remaining routers in its network segment with the update
using the IPv6 multicast address, FF02::5. In a scenario where the DR fails or
stops functioning, the BDR is used as the newly elected DR, and OSPF elects a
new BDR (, 2016). 

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