OSPF router id?

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Hi there ,can anyone clarify how this is worked out please-here is my
take on things
The priority takes 1st priority! higher number wins
Then its the IP addy o fthe router(locally)so even if the higher IP is
on a different network(say S1 is and e0 is
S1 IP is the router ID-even if it is just applied to the 192 network?
This has confused me today-I did a practice exam for Sem3 and had an
OSPF question-I got it correct by using the Serial link IP address to
work out who was the DR-however on the quiz at the end of the chapter
for OSPF routing ,my tutor explained that the serial link is not
In the question there were 3 routers -1,2,3
router 2 had the highest priority so it was DR
R1 had ip and a Loopback of
R3 had ip eo and a S1 of

I know R1 uses the loopback so its ID is however if that wasnt
there would R3 still be the BDR because it has the S1 ip addy?
Thanks alot ,

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