<P> Routing protocols like OSPF calculate the shortest route to a destination through the network based on an algorithm . The first routing protocol that was widely implemented, the Routing Information Protocol (RIP), calculated the shortest route based on hops, that is the number of routers that an IP packet had to traverse to reach the destination host . RIP successfully implemented dynamic routing, where routing tables change if the network topology changes . But RIP did not adapt its routing according to changing network conditions, such as data - transfer rate . Demand grew for a dynamic routing protocol that could calculate the fastest route to a destination . OSPF was developed so that the shortest path through a network was calculated based on the cost of the route, taking into account bandwidth, delay and load . Therefore OSPF undertakes route cost calculation on the basis of link - cost parameters, which can be weighted by the administrator . OSPF was quickly adopted because it became known for reliably calculating routes through large and complex local area networks . </P> <P> As a link state routing protocol, OSPF maintains link state databases, which are really network topology maps, on every router on which it is implemented . The state of a given route in the network is the cost, and OSPF algorithm allows every router to calculate the cost of the routes to any given reachable destination . Unless the administrator has made a configuration, the link cost of a path connected to a router is determined by the bit rate (1 Gbit / s, 10 Gbit / s, etc) of the interface . A router interface with OSPF will then advertise its link cost to neighbouring routers through multicast, known as the hello procedure . All routers with OSPF implementation keep sending hello packets, and thus changes in the cost of their links become known to neighbouring routers . The information about the cost of a link, that is the speed of a point to point connection between two routers, is then cascaded through the network because OSPF routers advertise the information they receive from one neighbouring router to all other neighbouring routers . This process of flooding link state information through the network is known as synchronisation . Based on this information, all routers with OSPF implementation continuously update their link state databases with information about the network topology and adjust their routing tables . </P> <P> An OSPF network can be structured, or subdivided, into routing areas to simplify administration and optimize traffic and resource utilization . Areas are identified by 32 - bit numbers, expressed either simply in decimal, or often in the same dot - decimal notation used for IPv4 addresses . By convention, area 0 (zero), or 0.0. 0.0, represents the core or backbone area of an OSPF network . While the identifications of other areas may be chosen at will; administrators often select the IP address of a main router in an area as the area identifier . Each additional area must have a connection to the OSPF backbone area . Such connections are maintained by an interconnecting router, known as an area border router (ABR). An ABR maintains separate link - state databases for each area it serves and maintains summarized routes for all areas in the network . </P> <P> OSPF detects changes in the topology, such as link failures, and converges on a new loop - free routing structure within seconds . </P>

When does an ospf router become an abr
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