Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 620

35 Open Shortest Path First (OSPFv2 and OSPFv3) This chapter describes how to configure and use Open Shortest Path First (OSPFv2 for IPv4) and OSPF version 3 (OSPF for IPv6). NOTE: The fundamental mechanisms of OSPF (flooding, DR election, area support, SPF calculations, and so on) are the same between OSPFv2 and OSPFv3. This chapter identifies and clarifies the differences between the two versions of OSPF. Except where identified, the information in this chapter applies to both protocol versions. OSPF protocol standards are listed in the Standards Compliance chapter. Topics: • Protocol Overview • OSPF Implementation • Configuration Information • Sample Configurations for OSPFv2 • Configuration Task List for OSPFv3 (OSPF for IPv6) Protocol Overview OSPF routing is a link-state routing protocol that calls for the sending of link-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 OSPF routers accumulate link-state information, they use the shortest path first (SPF) algorithm to calculate the shortest path to each node. OSPF routers initially exchange HELLO messages to set up adjacencies with neighbor routers. The HELLO process is used to establish adjacencies between routers of the AS. It is not required that every router within the AS areas establish adjacencies. If two routers on the same subnet agree to become neighbors through the HELLO process, they begin to exchange network topology information in the form of LSAs. In OSPFv2 neighbors on broadcast and NBMA links are identified by their interface addresses, while neighbors on other types of links are identified by RID. Autonomous System (AS) Areas OSPF operates 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 routing strategy. OSPF is an intra-AS (interior gateway) routing protocol, although it is capable of receiving routes from and sending routes to other ASs. You can divide an AS 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, called area border routers (ABRs), maintain separate databases for each area. Areas are a logical grouping of OSPF routers identified by an integer or dotted-decimal number. Areas allow you to further organize your routers within in the AS. One or more areas are required within the AS. Areas are valuable in that they allow sub-networks to "hide" within the AS, thus minimizing the size of the routing tables on all routers. An Open Shortest Path First (OSPFv2 and OSPFv3) 620