Neighbour Formation Stages in OSPF

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Neighbour Formation Stages in OSPF

1. Down stage	It is attempt stage/ starting stage         This is the first OSPF neighbour state. It means that no information (hellos) has been received from this neighbour, but hello packets can still be sent to the neighbour in this state.
2. Init	This state specifies that the router has received a hello packet from its neighbour, but the receiving router's ID was not included in the hello packet. When a router receives a hello packet from a neighbour, it should list the sender's router ID in its hello packet as an acknowledgment that it received a valid hello packet.
3. 2-Way stage	This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet.        At tilohe end of this stage, the DR and BDR for broadcast and non-broadcast multi-access networks are elected.
4. Ex-start	Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.        In this state, the routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation. The router with the higher router ID becomes the master and starts the exchange, and as such, is the only router that can increment the sequence number. Note that one would logically conclude that the DR/BDR with the highest router ID will become the master during this process of master-slave relation. Remember that the DR/BDR election might be purely by virtue of a higher priority configured on the router instead of highest router ID. Thus, it is possible that a DR plays the role of slave. And also note that master/slave election is on a per-neighbour basis.
5. Exchange	In the exchange state, OSPF routers exchange database descriptor (DBD) packets. Database descriptors contain link-state advertisement (LSA) headers only and describe the contents of the entire link-state database. Each DBD packet has a sequence number which can be incremented only by master which is explicitly acknowledged by slave. Routers also send link-state request packets and link-state update packets (which contain the entire LSA) in this state. The contents of the DBD received are compared to the information contained in the routers link-state database to check if new or more current link-state information is available with the neighbour.
6. Loading	In this state, the actual exchange of link state information occurs. Based on the information provided by the DBDs, routers send link-state request packets. The neighbour then provides the requested link-state information in link-state update packets. During the adjacency, if a router receives an outdated or missing LSA, it requests that LSA by sending a link-state request packet. All link-state update packets are acknowledged.
7. Full	In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers' databases are fully synchronized.          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 a broadcast network. Routers achieve the full state with their DR and BDR only. Neighbours always see each other as 2-way.

            In the Real environment, it will mainly stuck in the 2-Way stage and Ex-start.
Note:

• In a network all the router will sends route updates to all routers, so that the burden is very high on the network. To solve this DR and BDR is introduced. So that all the routers will send route updates to these DR and BDR only

Next Post is about DR and BDR and there selection process.....