1.
hello packets role
Neighbor Discovery: Automatically discover neighboring routers.
Neighbor established: complete parameter negotiation Hello packets to establish neighbor relations.
Neighbor Hold: by Keepalive mechanism to detect the neighbor running.
hello packet transmission time, depending on the link is the 10s and 30s, for periodically updating
There is also a hello packet deadtime for the aging time, this time must be four times the hello time, if not the neighbor relationship can not be established.
RTA can be seen in FIG hello packet to RTB, this time in the neighbor packet is null, a neighbor discovery, when RTB receipt of this message, the RTB state init.
RTA and RTB transmission neighbor hello neighbor found to be null for the synchronization, when the received hello RTB, will learn the source address of the packet in the RTA is their neighbors, then sends a new hello packet with to establish neighbor relations, this time packet neighbor to the RTA RouterID, when the RTA receive this message into the state 2way from init, while the RTA will send a neighbor hello message to the RTB RouterID when RTB receives init to enter from 2way state.
So how did you find OSPF neighbors do, because they do not know each other's IP address can not be so sure unicast form, it used to be in the form of a multicast hello packet, 224.0.0.5 destination address sent to NBMA network , that is, the network does not support multicast ip ospf need to manually specify the address of the neighbor.