D-Link DGS-6600-48TS Configuration Guide - Page 275

Volume 4-Layer 3 Configurations / Chapter 28-Open Shortest Path First Version 3 (OSPFv3) Chapter Overview The communication between the neighbor and the router is bidirectional. Neighbors will remain in this state if both the router itself and the neighbor are neither DR nor BDR. EXSTART This is the first step in creating an adjacency between the two routers. In this state the initial DD sequence number and the master is selected for the upcoming database exchange phase. EXCHANGE This is the database exchange phase. Additionally all neighbors in state EXCHANGE or higher (LOADING, FULL) participate in the flooding procedure. Starting from this state all packet types can be received inclusive flooded LS updates. LOADING The state is only entered if the Link-State Request list is not empty. In that case Link-State Request packets are sent out to fetch the more recent LSAs from the neighbors LS database. FULL The two routers are now fully adjacent. The connection will now appear in router-LSAs and networkLSAs. Only in this state real traffic will be routed between the two routers. Protocol Packet Processing Before a Hello packet is accepted, a number of criteria must be met. The diagram below shows the decision process for the acceptance of a Hello packet. The OSPF input process has already accepted the packet as described in the section "Message Format of OSPF for IPv6." Now the Hello Interval and the Router Dead Interval are checked. They must match the values set on the receiving interface. Next, the E and N bits in the Options field are examined. The settings of these bits must match the value set on the receiving interface. At this point, if all the criteria matched, the packet is accepted and the neighbor is identified by its Router ID. The router keeps a neighbor state table for each interface. If there is already a full adjacency with this neighbor, the Hello timer is simply reset. Otherwise, the state of this neighbor changes to initialize (Init). The router examines the list of neighbors proclaimed in the received Hello packet. If the router identifies its own Router ID in that list, bidirectional communication has been established, and the neighbor's state changes to two-way. The router decides whether to form an adjacency with this neighbor. If the interface is a point-to-point state, an adjacency is formed with this neighbor. On a transit link, if the router itself or this neighbor is the DR/BDR, an adjacency is formed. If the router decides not to form an adjacency, this neighbor stays in a two-way state. DGS-6600 Configuration Guide 275