Introducing the OSPF Protocol参考.ppt

Configuring OSPF Introducing the OSPF Protocol Link-State Protocols Link-State Data Structures Neighbor table: Also known as the adjacency database Contains list of recognized neighbors Topology table: Typically referred to as LSDB Contains all routers and their attached links in the area or network Identical LSDB for all routers within an area Routing table: Commonly named a forwarding database Contains list of best paths to destinations Link-State Routing Protocols Link-state routers recognize more information about the network than their distance vector counterparts. Each router has a full picture of the topology. Consequently, link-state routers tend to make more accurate decisions. Link-State Data Structure: Network Hierarchy Link-state routing requires a hierachical network structure that is enforced by OSPF. This two-level hierarchy consists of the following: Transit area (backbone or area 0) Regular areas (nonbackbone areas) OSPF Areas Area Terminology OSPF Adjacencies Routers discover neighbors by exchanging hello packets. Routers declare neighbors to be up after checking certain parameters or options in the hello packet. Forming OSPF Adjacencies Point-to-point WAN links: Both neighbors become fully adjacent. LAN links: Neighbors form a full adjacency with the DR and BDR. Routers maintain two-way state with the other routers (DROTHERs). Routing updates and topology information are passed only between adjacent routers. Once an adjacency is formed, LSDBs are synchronized by exchanging LSAs. LSAs are flooded reliably throughout the area (or network). OSPF Calculation Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to the link-state database as follows: Every router in an area has the identical link-state database. Each router in the area places itself into the root of the tree that is built. The best path is calculated with respect to the lowest total cost of links to a specific destination. Best routes are put i