[单选] Open Shortest Path First (OSPF) was designed as an （） gateway protocol, for use in an autonomous system such as a local area network (LAN). It implements Dijkstra’s algorithm, also known as the （） path first algorithm. As a link-state routing protocol it was based on the link-state algorithm developed for the ARPANET in 1980 and the IS-IS routing protocol. OSPF was first standardised in 1989 as RFC 1131, which is now known as OSPF version 1.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 （）, 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. （） 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.As a link state routing protocol, OSPF maintains link state （请作答此空）, 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.OSPF has become a popular dynamic routing protocol. Other commonly used dynamic routing protocols are the RIP and the Border Gateway Protocol (BGP). Today routers support at least one interior gateway protocol to advertise their routing tables within a local area network. Frequently implemented interior gateway protocols besides OSPF are RIP, IS-IS, and the proprietary Interior Gateway Routing Protocol (IGRP) by Cisco.
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