Explanation & Hint:
- Two-Way: This is the state where the DR/BDR election is conducted. In this state, OSPF routers have acknowledged each other’s presence through Hello packets. This acknowledgement is necessary before proceeding with the election process. The election of the Designated Router (DR) and Backup Designated Router (BDR) happens only on multi-access networks like Ethernet. The purpose of the DR and BDR is to reduce the amount of OSPF traffic and to streamline the process of exchanging route information.
- Init: In this state, a router has received a Hello packet from another router but has not established two-way communication yet. The router that sent the Hello packet is listed in the receiving router’s neighbor list, but the receiving router is not yet listed on the sender’s list. This is more of a preliminary state before any significant OSPF interactions occur.
- ExStart: Once the DR and BDR are elected, and a router starts forming an adjacency with its neighbors, it enters the ExStart state. In this phase, routers establish a master-slave relationship and decide on initial sequence numbers for exchanging OSPF link-state advertisements (LSAs). This state lays the groundwork for the actual exchange of routing information.
- Exchange: In the Exchange state, OSPF routers exchange database descriptor (DBD) packets. These packets contain summaries of the router’s link-state database (LSDB). The exchange of DBD packets allows routers to determine which link-state information is new or outdated, necessitating further information exchange to ensure all routers in an OSPF area have synchronized LSDBs.
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