D-Link DFL-260E User Manual for DFL-260E - Page 199
OSPF Concepts, Overview, The Autonomous System, Link-state Routing
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4.5.2. OSPF Concepts Chapter 4. Routing and a brief explanation is given here. Routing metrics are the criteria that a routing algorithm will use to compute the "best" route to a destination. A routing protocol relies on one or several metrics to evaluate links across a network and to determine the optimal path. The principal metrics used include: Path length Item Bandwidth Load Delay The sum of the costs associated with each link. A commonly used value for this metric is called "hop count" which is the number of routing devices a packet must pass through when it travels from source to destination. The traffic capacity of a path, rated by "Mbps". The usage of a router. The usage can be evaluated by CPU utilization and throughput. The time it takes to move a packet from the source to the destination. The time depends on various factors, including bandwidth, load, and the length of the path. 4.5.2. OSPF Concepts Overview Open Shortest Path First (OSPF) is a routing protocol developed for IP networks by the Internet Engineering Task Force (IETF). The NetDefendOS OSPF implementation is based upon RFC 2328, with compatibility to RFC 1583. OSPF is not available on all D-Link NetDefend models The OSPF feature is only available on the NetDefend DFL-860E, 1660, 2560 and 2560G. OSPF functions by routing IP packets based only on the destination IP address found in the IP packet header. IP packets are routed "as is", in other words they are not encapsulated in any further protocol headers as they transit the Autonomous System (AS). The Autonomous System The term Autonomous System refers to a single network or group of networks with a single, clearly defined routing policy controlled by a common administrator. It forms the top level of a tree structure which describes the various OSPF components. In NetDefendOS, an AS corresponds to an OSPF Router object. This must be defined first when setting up OSPF. In most scenarios only one OSPF router is required to be defined and it must be defined separately on each NetDefend Firewall involved in the OSPF network. This NetDefendOS object is described further in Section 4.5.3.1, "OSPF Router Process". OSPF is a dynamic routing protocol as it quickly detects topological changes in the AS (such as router interface failures) and calculates new loop-free routes to destinations. Link-state Routing OSPF is a form of link-state routing (LS) that sends Link-state Advertisements (LSAs) to all other routers within the same area. Each router maintains a database, known as a Link-state Database, which maps the topology of the autonomous system (AS). Using this database, each router constructs a tree of shortest paths to other routers with itself as the root. This shortest-path tree yields the best route to each destination in the AS. 199