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Embed code for: 23.7 Introducing OSPF
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23.7 Introducing OSPF
Introducing OSPF Section 23: Implementing Dynamic Routing OSPF is a linkstate routing protocol. You can think of a link as an interface on a router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the subnet mask, the type of network to which it is connected, the routers that are connected to that network, and so on. The collection of all of these link states forms a linkstate database. OSPF was developed based on an open standard and is supported by several router manufacturers. OSPF is widely used as an IGP, especially in large network environments. A router sends LSA packets immediately to advertise its state when there are state changes, and the packets are sent periodically as well (every 30 minutes by default). Information about attached interfaces, metrics that are used, and other variables are included in OSPF LSAs. As OSPF routers accumulate linkstate information, they use the SPF algorithm to calculate the shortest path to each node. A topological (linkstate) database is, essentially, an overall picture of the networks in relation to the routers. The topological database contains the collection of LSAs that was received from all routers in the same area. Because routers within the same area share the same information, they have identical topological databases. OSPF can operate within a hierarchy. The largest entity within the hierarchy is the AS, which is a collection of networks under a common administration that share a common routing strategy. An AS can be divided into a number of areas, which are groups of contiguous networks and attached hosts. The figure shows an example of an OSPF hierarchy. OSPF uses a twolayer network hierarchy that has two primary elements: AS: An AS consists of a collection of networks under a common administration that share a common routing strategy. An AS, sometimes called a domain, can be logically subdivided into multiple areas. Area: An area is a grouping of contiguous networks. Areas are logical subdivisions of the AS. Within each AS, a contiguous backbone area must be defined. All other nonbackbone areas are connected off the backbone area. The backbone area is the transition area because all other areas communicate through it. For OSPF, the nonbackbone areas can be additionally configured as stub areas, totally stubby areas, or NSSAs to help reduce the sizes of the linkstate database and the routing table. Areas such as NSSAs, totally stubby areas, and stub areas are beyond the scope of this text. Routers that operate within the twolayer network hierarchy have different routing entities and different functions in OSPF. The following are some examples based on the figure: Router B is the backbone router. The backbone router provides connectivity between different areas. Routers C, D, and E are ABRs. ABRs attach to multiple areas, maintain separate linkstate databases for each area to which they are connected, and route traffic that is destined for or arriving from other areas. Routers F, G, and H are nonbackbone, internal routers. Nonbackbone, internal routers are aware of the topology within their respective areas and maintain identical linkstate databases about the areas. Depending on the configuration of the OSPF nonbackbone area (stub area, totally stubby area, or NSSA), the ABR advertises a default route to the nonbackbone, internal router. The nonbackbone, internal router uses the default route to forward all interarea or interdomain traffic to the ABR. Router A is the ASBR that connects to an external routing domain, or AS. Router I is a router that belongs to another routing domain, or AS. Up Next: OSPF Adjacencies