D-Link DGS-6600-48T Configuration Guide - Page 217

Volume 4-Layer 3 Configurations / Chapter 23-Open Shortest Path First (OSPF) Chapter Overview Chapter 23 Open Shortest Path First (OSPF) Chapter Overview The following topics are included in this chapter, please go to the topic for more detailed information: • An Introduction to OSPF • An Introduction to OSPF • OSPF Configuration Commands • Basic Commands and Functions • Generating a Default Route • Redistributing Routes to OSPF • Displaying Border Routers • Restarting OSPF • Configuration Examples • OSPFv2 Configuration (Basic) Example • OSPFv2 Configuration Example 2 • List of Constants and Default Settings An Introduction to OSPF OSPF is an interior routing protocol that operates within the scope of a domain. A domain is also an autonomous system. An OSPF domain is divided into several sub-domains, which are called areas. Routers within the same area need to maintain a database that defines the topology of the area, but have no need to understand details about the topology outside their own area. Due to this hierarchical structure, OSPF is a routing protocol that is suitable for deploying on large networks. The division of areas in OSPF is based on a two level structure, with the backbone area being on the first level and the non-backbone areas being on the second level. Each OSPF domain consists of a single backbone area and all other areas in the OSPF domain need to connect to the backbone area. Each area in the OSPF domain needs to have a unique ID. OSPF always identifies the backbone area as zero. Routing information in non-zero areas can be exchanged with the zero area but can not be exchanged with other non-zero areas. Typically, summarized routes are exchanged between the different areas. This design reduces the number of routes that are propagated across areas and maintains the simplicity of the protocol operation. The router that connects the non-zero area to the zero area is the Area Border Router (ABR). ABRs are responsible for carrying out inter-area routing. An AS Border Router (ASBR) connects the area within an AS to the external network. ASBRs are responsible for redistributing the external routes to the OSPF AS and vice versa. Thus, the ASBR carries out inter-AS routing. The router within the same area carries out the intra-area routing. OSPF routers exchange routing information using Link State Advertisements (LSA). The exchanged information is divided into the following types: • Router LSA (Type 1) • Network LSA (Type 2) DGS-6600 Configuration Guide 217