OSPF Network Types :
Point-to-point networks : such as a T1 or subrate link, connect a single pair of routers. Valid neighbors on
point-to-point networks will always become adjacent. The destination address of OSPF packets on these
networks will always be the reserved class D address 184.108.40.206, known as AllSPFRouters.
Broadcast networks : such as Ethernet, Token Ring, and FDDI, might be better defined as broadcast multiaccess
networks to distinguish them from NBMA networks. Broadcast networks are multi-access in that
they are capable of connecting more than two devices, and they are broadcast in that all attached devices
can receive a single transmitted packet. OSPF routers on broadcast networks will elect a DR and a BDR. Hello packets are
multicast with the AllOSPFRouters destination address 220.127.116.11, as are all OSPF packets originated by the
DR and BDR. The destination Media Access Control (MAC) identifier of the frames carrying these
packets is 0100.5E00.0005. All other routers will multicast link state update and link state
acknowledgment packets to the reserved class D address 18.104.22.168, known as
AllDRouters. The destination MAC identifier of the frames carrying these packets is 0100.5E00.0006.
NBMA networks : such as X.25, Frame Relay, and ATM, are capable of connecting more than two routers
but have no broadcast capability. A packet sent by one of the attached routers would not be received by
all other attached routers. As a result, extra configuration may be necessary for routers on these networks
to acquire their neighbors. OSPF routers on NBMA networks elect a DR and BDR, and all OSPF packets
Point-to-multipoint networks : are a special configuration of NBMA networks in which the networks are
treated as a collection of point-to-point links. Routers on these networks do not elect a DR and BDR, and
because the networks are seen as point-to-point links, the OSPF packets are multicast.