OSPF network types
1, point to point
3, one to multi points broadcast
4, one to multi points no broadcast
5, no broadcast
Especially, point to point and one to multi points* do not need select DR and BDR, because, one to multi points (both broadcast and no broadcast) will work in the way like several point-to-point links, there is no need to select DR and BDR. Especially, router with the highest priority will become DR and the router with the second highest priority will become BDR. In Juniper router has default priority of 128, in Cisco router has default priority of 1.
On the other side, both type 2 and type 5 need select DR and BDR in order to reduce the numbers of ospf packets (hello, lsa, etc) in the network.
Normally ospf build up adjacency by broadcasting hello packets to 126.96.36.199, but in some noethernet network (mostly in frame relay network), broadcasting is not applicable, hello packets are sent out with unique ip address of the peer side.
Network area types
1, Backbone area ( area 0 ) , can receive all LSA types information
2, Standard area, can receive type 3 info from ABR (Area border router)
3, Stub area, can receive normaly LSA type 3 info and a default route as subsitute for all external routes
4, Totally stub area, can receive only one LSA type 3 as default router towards outside of the area
5, Not so stub area, can work as stub area or totally stub area, BUT, can send external routes from ASBR as type 5 LSA to the other area.
type 1: router
type2: net route, generated and sent only by DR
type3, net summary route, generated and sent only by ABR in order to reduce the numbers of routes sent to the other area.
type4: serve to advertise the presence of an autonomous system boundary router (ASBR).
type 5:external route that send to other network area from NSSB(not so stub area)
type 7: external route
Especially, type 1 and type 2 does not cross area border.