Routers running OSPF transition through several states before establishing an adjacency. The routers exchange different types of packets during these states. This exchange of messages allows all routers that establish an adjacency to have a consistent view of the network. Additional changes to the current network are simply sent out as incremental updates. The different states are the Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full states, as described below:
- The Down state is the starting state for all OSPF routers. However, the local router may also show a neighbour in this state when no Hello packets have been received within the specified router dead interval for that interface.
- The Attempt state is valid only for OSPF neighbours on NBMA networks. In this state, a Hello has been sent but no information has been received from the statically configured neighbour within the dead interval; however, some effort is being made to establish an adjacency with this neighbour.
- The Init state is reached when an OSPF router receives a Hello packet from a neighbour but the local RID is not listed in the received Neighbor field. If OSPF Hello parameters, such as timer values, do not match, then OSPF routers will never progress beyond this state.
- The 2-Way state indicates bidirectional communication (each router has seen the other’s Hello packet) with the OSPF neighbour(s). In this state, the local router has received a Hello packet with its own RID in the Neighbor field and Hello packet parameters are identical on the two routers. At this state, a router decides whether to become adjacent with this neighbour. On Multi-Access networks, the DR and the BDR are elected during this phase.
- The Exstart state is used for the initialisation of the database synchronisation process. It is at this stage that the local router and its neighbour establish which router is in charge of the database synchronisation process. The Master and Slave are elected in this state, and the first sequence number for DBD exchange is decided by the Master in this stage.
- The Exchange state is where routers describe the contents of their databases using DBD packets. Each DBD sequence is explicitly acknowledged, and only one outstanding DBD is allowed at a time. During this phase, LSR packets are also sent to request a new instance of the LSA. The M (More) bit is used to request missing information during this stage. When both routers have exchanged their complete databases, they will both set the M bit to 0.
- In the Loading state, OSPF routers build an LSR and Link State Retransmission list. LSR packets are sent to request the more recent instance of an LSA that has not been received during the Exchange process. Updates that are sent during this phase are placed on the Link State Retransmission list until the local router receives an acknowledgement. If the local router also receives an LSR during this phase, it will respond with a Link State Update that contains the requested information.
- The Full state indicates that the OSPF neighbours have exchanged their entire databases and both agree (i.e., have the same view of the network). Both neighbouring routers in this state add the adjacency to their local database and advertise the relationship in a Link State Update packet. At this point, the routing tables are calculated, or recalculated if the adjacency was reset. Full is the normal state for an OSPF router. If a router is stuck in another state, it’s an indication that there are problems in forming adjacencies. The only exception to this is the 2-Way state, which is normal in Broadcast and Non-Broadcast Multi-Access networks where routers achieve the Full state with their DR and BDR only. Other neighbours always see each other as 2-Way.
In order for an OSPF adjacency to be established successfully, certain parameters on both routers must match. These parameters include the following:
- The interface MTU values (can be configured to be ignored)
- The Hello and Dead timers
- The Area ID
- The Authentication type and password
- The Stub Area flag
- Compatible network types
These parameters will be described as we progress through this module. If these parameters do not match, the OSPF adjacency will never fully establish.
NOTE: In addition to mismatched parameters, it is also important to remember that on a Multi-Access network, if both routers are configured with a priority value of 0, then the adjacency will not be established. The DR must be present on such network types.