OSPF Basic Topology and the Link State Database (LSDB)
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OSPF is designed to facilitate routing in both smaller and larger autonomous systems (ASes). To this end, the protocol supports two topologies. When there is only a small number of routers, the entire AS is managed as a single entity. This doesn't have a specific name, but I call it OSPF basic topology to convey the simple nature of the topology, and to contrast it with the hierarchical topology we will explore in the topic following this one.
When OSPF basic topology is used, all the routers in the AS function as peers. Each router communicates routing information with each other one, and each maintains a copy of the key OSPF data structure: the link-state database (LSDB). The LSDB is essentially a computerized representation of the topology of the autonomous system. It is the method by which routers see the state of the links in the autonomous systemthus the name link-state database (and for that matter, the name of the entire class of link-state algorithms of which OSPF is a part.)
The LSDB is a bit hard to visualize, but is best viewed as a set of data that is equivalent to a graphical picture showing the topology of an autonomous system. In such a diagram, we typically show routers and networks as nodes, and connections between routers and networks as lines that connect them. The OSPF LSDB takes that information and puts it into a table, to allow a router to maintain a virtual picture of all the connections between routers and networks in the AS.
The LSDB therefore indicates which routers can directly reach which other routers, and also which networks each can reach. Furthermore, it stores for each of these links a cost to reach the network. This cost is an arbitrary metric that can be set up based on any criteria important to the administrator. OSPF is not restricted to the overly-simple hop-count metric used in RIP.
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