Neighbor States and Problems of a Neighbor

OSPF routers go through several neighbor states as they establish their adjacency. The states are:

Down state: In this state, there is no communication between the routers.

Init state: In this state, a router has sent a Hello packet but has not received any reply from its neighbor.

Two-way state: In this state, the router has received a Hello packet from its neighbor, and the neighbor’s Router ID is present in the received packet.

Exstart state: In this state, the router with the higher Router ID becomes the master and sends a Database Descriptor (DBD) packet to the other router.

Exchange state: In this state, the routers exchange DBD packets and link-state request (LSR) packets.

Loading state: In this state, the routers exchange link-state update (LSU) packets to synchronize their link-state databases.

Full state: In this state, the routers have a complete and synchronized link-state database and are ready to exchange routing updates.

Problems with OSPF neighbors can occur due to various reasons, such as:

Network connectivity issues: If the network link between two routers goes down, the OSPF adjacency between them will also fail.

Mismatched parameters: If the OSPF parameters such as the OSPF area ID, network type, authentication type, or router ID are not the same on both routers, they will not be able to form an adjacency.

MTU mismatch: If the Maximum Transmission Unit (MTU) size on the network link is not the same on both routers, it can cause OSPF neighborship issues.

Authentication issues: If the OSPF authentication key is not the same on both routers, the OSPF adjacency will not form. Routing loops: If there is a routing loop in the network, it can cause OSPF neighbors to continuously re-establish their adjacency, leading to instability in the network.