Dell Force10 S4820T S4820T Configuration Guide - Page 689

Area Types The Backbone of the network is Area 0. It is also called Area 0.0.0.0 and is the core of any Autonomous System (AS). All other areas must connect to Area 0. Areas can be defined in such a way that the backbone is not contiguous. In this case, backbone connectivity must be restored through virtual links. Virtual links are configured between any backbone routers that share a link to a non-backbone area and function as if they were direct links. An OSPF backbone is responsible for distributing routing information between areas. It consists of all Area Border Routers, networks not wholly contained in any area, and their attached routers. The Backbone is the only area with an default area number. All other areas can have their Area ID assigned in the configuration. Figure 33-1 shows Routers A, B, C, G, H, and I are the Backbone. A Stub Area (SA) does not receive external route information, except for the default route. These areas do receive information from inter-area (IA) routes. Note that all routers within an assigned Stub area must be configured as stubby, and no 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. Stubby areas cannot be traversed by a virtual link. A Not-So-Stubby Area (NSSA) can import AS external route information and send it to the Backbone. It cannot received external AS information from the Backbone or other areas. It can be traversed by a virtual link. Totally Stubby Areas are referred to as No Summary areas in FTOS. Networks and Neighbors As a link-state protocol, OSPF sends routing information to other OSPF routers concerning the state of the links between them. The state (up or down) of those links is important. Routers that share a link become neighbors on that segment. OSPF uses the hello protocol as a neighbor discovery and keep alive mechanism. After two routers are neighbors, they may proceed to exchange and synchronize their databases, which creates an adjacency. Router Types Router types are attributes of the OSPF process. A given physical router may be a part of one or more OSPF processes. For example, a router connected to more than one area, receiving routing from a BGP process connected to another AS acts as both an Area Border Router and an Autonomous System Router. Each router has a unique ID, written in decimal format (A.B.C.D). The router ID does not have to be associated with a valid IP address. However, Dell Force10 recommends that the router ID and the router's IP address reflect each other, to make troubleshooting easier. Open Shortest Path First (OSPFv2) | 689