Before knowing about the DR and BDR election we need to know what is the use of this election on LAN but not Point to point?
On point to point there always only two devices to communicate when one device transmits the other always listens to it but coming to LAN think that what happens when a router knows about one update so it need to send it to all of its neighbors and the receiving router receives and also wants its neighbors to know about the update so what will happen if it repeats continues on LAN? It will consume the routers processing power so limit this there will DR and BDR for the LAN which whenever there is an update then sender directly send it to DR and BDR multicast address of 224.0.0.6 and then the DR sends it to all the neighbors with multicast address 224.0.0.5.so this will reduce unnecessarily propagating of ospf updates over the shared media.
Remember on LANS
DR/BDR listens to 224.0.0.6 multicast address
OTHER listens to 224.0.0.5 multicast address
let me show how this LSA updates being propagates in a LAN environment by capturing packets at one of the connection to the switch.
you can see from the above pic the hello packets are sending to multicast address 224.0.0.5 and LS updates are being sent to multicast address of 224.0.0.6
How election invokes?
On LAN, DR/BDR selected on the following rule
Highest IP address on the LAN becomes DR
Second highest IP address becomes BDR
and also remember router-id beats loopback address beats ip address
here i continue my discussion with ip addresses,consider below figure which 4 routers configured and became ospf neighbours,Initially the highest ip address on the LAN becomes DR
if we see on router 10.0.0.4 by issuing "show ip ospf neighbors" we observe the following
From the above picture you can observe 10.0.0.4 became DR being highest ip address on the LAN
Here i'd like to show how the election process works with different scenarios
First i make DR down by shutting down the lan interface
when i make the DR down then BDR becomes DR as you can see from the below pic
From the above pic you can see that being BDR ,10.0.0.3 became DR and to elect BDR, election is invoked among the remaining routers so 10.0.0.2 became BDR now.
so what happens when DR comes online back? it doesn't become DR again because there are already DR and BDR exist so no election is invoked.
