OSPF neighbor states
Before establishing a neighbor relationship, OSPF routers go through several state changes (up to eight possible states):
1. Down state – no Hellos have been received on the interface. All OSPF routers begin in this state.
2. Attempt state – neighbors must be manually configured. This state is used only in nonbroadcast multi-access (NBMA) networks.
3. Init state – a router has received a Hello message from the other OSFP router, but the two-way conversation has not yet been established.
4. 2-way state – the neighbor has received the Hello message and replied with a Hello message of his own.
5. Exstart state – beginning of the LSDB exchange between both routers. Routers will start to exchanging link state information.
6. Exchange state – DBD (Database Descriptor) packets are exchanged. DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged.
7. Loading state – one neighbor sends LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process.
8. Full state – both routers have synchronized the link state database and are fully adjacent with each other. OSPF routing can now begin.