It has a virtual link feature that can be used to create an adjacency tunnel across multiple hops. The protocol may therefore be considered a link layer protocol, but is often also attributed to the application layer in the TCP/IP model. OSPF multicast IP packets never traverse IP routers, they never travel more than one hop. For non-broadcast networks, special provisions for configuration facilitate neighbor discovery. It reserves the multicast addresses 224.0.0.5 (IPv4) and FF02::5 (IPv6) for all SPF/link state routers (AllSPFRouters) and 224.0.0.6 (IPv4) and FF02::6 (IPv6) for all Designated Routers (AllDRouters). OSPF also uses multicast addressing for distributing route information within a broadcast domain. OSPF implements its own transport error detection and correction functions. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP) and the Border Gateway Protocol (BGP). It encapsulates its data directly in IP packets with protocol number 89. OSPF runs over IPv4 and IPv6, but does not use a transport protocol such as UDP or TCP. An ABR maintains separate link-state databases for each area it serves and maintains summarized routes for all areas in the network. Such connections are maintained by an interconnecting router, known as an area border router (ABR). Each additional area must have a connection to the OSPF backbone area. 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. By convention, area 0 (zero), or 0.0.0.0, represents the core or backbone area of an OSPF network. Areas are identified by 32-bit numbers, expressed either simply in decimal, or often in the same octet-based dot-decimal notation used for IPv4 addresses. OSPF divides the network into routing areas to simplify administration and optimize traffic and resource utilization. This provides a dynamic process of traffic load balancing between routes of equal cost. Cost factors may be the distance of a router ( round-trip time), data throughput of a link, or link availability and reliability, expressed as simple unitless numbers. The OSPF routing policies for constructing a route table are governed by link metrics associated with each routing interface. It computes the shortest-path tree for each route using a method based on Dijkstra's algorithm. OSPF detects changes in the topology, such as link failures, and converges on a new loop-free routing structure within seconds. The topology is presented as a routing table to the internet layer which routes packets based solely on their destination IP address. It gathers link state information from available routers and constructs a topology map of the network. OSPF is an interior gateway protocol (IGP) for routing Internet Protocol (IP) packets within a single routing domain, such as an autonomous system. OSPF supports the Classless Inter-Domain Routing (CIDR) addressing model. The updates for IPv6 are specified as OSPF version 3 in RFC 5340 (2008). Originally designed in the 1980s, OSPF version 2 is defined in RFC 2328 (1998). IS-IS, another LSR-based protocol, is more common in large service provider networks. OSPF supports Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6) networks and is widely used in large enterprise networks. The topology is presented as a routing table to the internet layer for routing packets by their destination IP address. OSPF gathers link state information from available routers and constructs a topology map of the network. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS). Open Shortest Path First ( OSPF) is a routing protocol for Internet Protocol (IP) networks.
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