OSPFv2 Neighbor Adjacency States

OSPF distinguishes between the terms neighbor and adjacency, where neighbor refers to any routers exchanging valid hello packets and adjacency meaning routers that exchange routing information.

So, a router running OSPF may have 3 neighbors in total, but only 2 are adjacent. At the end of this post I will include an example of such a scenario.

So, on to the various eight states an OSPF neighbor relationship can be in. These are the state of the OSPF State Machine a google search for "OSPF State Machine" will yield a great deal of graphical and in-depth explanations - so I will just not try and do that kind of stuff and just list the states with a few clarifying notes on each of them.

Down
The neighbor always starts from the down state and moves to either attempt or init states depending on the OSPF network type. The neighbor will also revert to the down state if the dead interval expires or the neighbor adjacency gets cleared.

Attempt
If the neighbor is manually configured it will enter the attempt state, which indicates that the local router has sent unicast hellos to the configured neighbor, but has not heard any hellos in return. This states is only seen on NBMA network types (like non-broadcast multi-access and point-to-multipoint non-broadcast networks).
This is a transitory state.

Init
The init state indicates that the local router has received a hello packet from a neighbor, but without its own RID contained in the packet. This state occurs on network types that do not enter the attempt state.
This is a transitory state.

2-way
Neighbor is stated as 2-Way when a hello packet is received containing the RID of the local router; this would indicate that the neighbor has received a hello from the local router and thus bidirectional communication is possible.
This state is both a stable and a transitory state depending on the network type. Broadcast networks with will have a DR, a BDR and probably quite a few DROther routers, where the DR and BDR are the only ones to have full adjacencies with all routers - the DROther routers will have full adjacency with the DR/BDR and 2-way state for the rest of the neighbors.

ExStart
If the routers are not meant to be stuck in the 2-way state they will have to negotiate a master/slave relationship and agree on a starting sequence number for subsequent Database Description packets (DBDs). This is basically done by exchanging empty DBDs and comparing the RIDs where the router with the highest RID becomes the master.
(The masters role is to send DBDs first to slave, which in turn allows the slave to reply with a DBD. If the master is done sending DBDs, but the slave is not done yet, the master must keep sending DBDs until the slave indicates it is complete - as the slave is not permitted to send DBDs on its own.)

Exchange
After the master/slave relationship is negotiated the routers will enter the exchange state and start sending DBDs containing their LSAs. The router builds a list of missing LSAs based on the received DBDs.

Loading
After the DBD exchange is over the router will download LSAs from its neighbor based on the list made during the exchange state. This is a transitory state.

Full
When all the LSAs have been downloaded the neighbor adjacency will enter into the full state, which is a steady state for all routers exchanging LSAs - meaning all routers except the DROthers.

Now, on to the OSPF neighbor state example. I will be using the network topology shown in the diagram below and the example will show the outputs from the perspective of the router R3.

OSPFv2 four node broadcast topology

The topology shows 4 routers connected over a shared ethernet segment (130.1.255.0/24). Router R1 is the DR and R2 is the BDR. The rest are DROther.

R3 show ip ospf interface brief

Notice the state of DROTH from the output above and that the Nbrs F/C field shows 2/3 - meaning that 2 routers are adjacent and that there are 3 neighbors in total. A router must be a neighbor to be adjacent, which means that there are 2 out of 3 routers in the full state on this interface.

Below, we see the neighbors of R3 and verify that there are three neighbors in total, where two are in the FULL state and the last is in the 2WAY state.

R3 show ip ospf neighbor