Dell MX9116n EMC SmartFabric OS10 User Guide - Page 608

Open shortest path first OSPF routing is a link-state routing protocol that allows sending link-state advertisements (LSAs) to all other routers within the same autonomous system (AS) area. OSPF LSAs include information about attached interfaces, metrics used, and other attributes. OSPF routers accumulate link-state information, and use the shortest path first (SPF) algorithm to calculate the shortest path to each node. Autonomous system areas OSPF operates in a hierarchy. The largest entity within the hierarchy is the autonomous system (AS). The AS is a collection of networks under a common administration that share a common routing strategy. OSPF is an intra-AS, Interior Gateway Routing Protocol (IGRP) that receives routes from and sends routes to other AS. You can divide an AS into several areas, which are groups of contiguous networks and attached hosts administratively grouped. 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 that an integer or dotted-decimal number identifies. Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas allow subnetworks to hide within the AS-minimizing the size of the routing tables on all routers. An area within the AS may not see the details of another area's topology. An area number or the router's IP address identifies AS areas. Areas, networks, and neighbors The backbone of the network is Area 0, also called Area 0.0.0.0, the core of any AS. All other areas must connect to Area 0. An OSPF backbone distributes routing information between areas. It consists of all area border routers and networks not wholly contained in any area and their attached routers. The backbone is the only area with a default area number. You configure all other areas Area ID. If you configure two nonbackbone areas, you must enable the B bit in OSPF. Routers, A, B, C, G, H, and I are the backbone, see Autonomous system areas. • A stub area (SA) does not receive external route information, except for the default route. These areas do receive information from interarea (IA) routes. • A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone as type-7 LSA. • Totally stubby areas are also known as no summary areas. Configure all routers within an assigned stub area as stubby and do not generate LSAs that do not apply. For example, a Type 5 LSA is intended for external areas and the stubby area routers may not generate external LSAs. A virtual link cannot traverse stubby areas. Networks and neighbors 608 Layer 3