620 | Open Shortest Path First (OSPFv2 and OSPFv3)w w w . d e l l . c o m | s u p p o r t . d e l l . c o mProtocol OverviewOpen Shortest Path First (OSPF) routing is a link-state routing protocol that calls for the sending ofLink-State Advertisements (LSAs) to all other routers within the same Autonomous System (AS) Areas.Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs. As OSPFrouters accumulate link-state information, they use the SPF algorithm (Shortest Path First algorithm) tocalculate the shortest path to each node.OSPF routers initially exchange HELLO messages to set up adjacencies with neighbor routers. TheHELLO process is used to establish adjacencies between routers of the AS. It is not required that everyrouter within the Autonomous System areas establish adjacencies. If two routers on the same subnet agreeto become neighbors through the HELLO process, they begin to exchange network topology informationin the form of Link State Advertisements (LSAs).OSPFv3 runs on a per-link basis instead of on a per-IP-subnet basis. All neighbors on all link types areidentified by Router ID (RID). In OSPFv2 neighbors on broadcast and NBMA links are identifiedby their interface addresses, while neighbors on other types of links are identified by RID.OSPFv3 removes this inconsistency, and all neighbors on all link types are identified by RID.Autonomous System (AS) AreasOSPF operate in a type of hierarchy. The largest entity within the hierarchy is the autonomous system(AS), which is a collection of networks under a common administration that share a common routingstrategy. OSPF is an intra-AS (interior gateway) routing protocol, although it is capable of receiving routesfrom and sending routes to other ASs.An AS can be divided into a number of areas, which are groups of contiguous networks and attached hosts.Routers with multiple interfaces can participate in multiple areas. These routers, Area Border Routers(ABRs), maintain separate databases for each area. Areas are a logical grouping of OSPF routers identifiedby an integer or dotted-decimal number.Areas allow you to further organize your routers within in the AS. One or more areas are required withinthe AS. Areas are valuable in that they allow sub-networks to "hide" within the AS, thus minimizing thesize of the routing tables on all routers. An area within the AS may not see the details of another Area'stopology. AS areas are known by their area number or the router’s IP address.Note: OSPFv3 is not backward-compatible with OSPFv2; they can co-exist. To use OSPFwith both IPv4 and IPv6, you must run both OSPFv2 and OSPFv3.
