OSPF Basics

OSPF (Open Shortest Path First) is an advanced routing protocol used to determine where to route packets from one IGP (Interior Gateway Protocol) network. This type of routing protocol is considered a link state protocol. Link-state routing differs from distance vector routing (eg RIP) in many ways, but there are some similarities. In addition to routing data in networks, these two types of protocols to send routing updates to its neighboring routers in turn forward its neighbor router these changes to its neighbors and so on.

There are many more differences than similarities between the link status and distance vector protocols, however. Routers using link state announces full details of the internal network to all other routers. After all routers to complete this process, each router will have accurate information about the same internal network. This information is stored in a database on each router in what's called a link state database or (LSDB). Similar to the way every car traveling to a destination on the interstate itself may have exactly the same U.S. map. However, each car can travel to a different destination, and each vehicle will travel in a certain direction, depending on where they are in the U.S. so that the map is used to determine the best way to reach a specific destination location. OSPF works the same way, the LSDB is the map and depending on where you are and where the router is trying to send data, use the LSDB to determine the best route to your destination.

For each router to learn about what IP and networks are in the internal network, OSPF uses what is called Link-state advertisements (LSA). The two main types of LSA is the Router LSA and LSA liaison. The LSA Router includes identification information of the router, interface IP addresses, the status of each interface, and the cost or indicator associated with each interface. The LSA Liaison identifies each link (subnet) and routers that are connected to that link. The link state (up or down) SIA also included in the LSA. OSPF LSA refloods default every 30 minutes, unless the network changes, such as interfaces to occur.