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 (Cisco.com, 2016).

 

2.2.2.8 Shortest Path First Algorithm

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Consider
an AS with link–state information shown in Figure 2.3. The cost metric (CST)
assigned on each router interface to every network (NET) indicating the choice
of using that interface is given by the arabic numerals shown in the figure. In
order to build a LSDB from which the shortest path to every network can be
calculated, each router is expected to receive a valid LSA from its neighbors
in the network. Before the LSDB is built, each router will forward an LSA
containing link–state information (cost) on all the networks that are directly
attached to it. When a router receives an LSA from other routers, it will send
this LSA to its neighbors. When the network is converged, each router in the AS
shown in Figure 2.3 will have the LSDB shown in Table 2.1.