Open Shortest Path First (OSPF) is a link-state routing protocol defined in RFC 2328. It is designed to be run as an Interior Gateway Protocol (IGP) to a single Autonomous System (AS). In a link-state routing protocol, each router maintains a database of router advertisements called Link State Advertisements (LSAs). LSAs for routers within the AS consist of a router, its attached networks, and their configured costs. An OSPF cost is a unitless metric that indicates the preference of using a link. There are also LSAs for summarized routes and routes outside of the AS.
The router distributes its LSAs to its neighboring routers. LSAs are gathered into a database called the link state database (LSDB). By synchronizing LSDBs between all neighboring routers, each router has each other router's LSA in its database. Therefore, every router has the same LSDB. From the LSDB, entries for the router's routing table are calculated using the Dijkstra algorithm to determine the least cost path, the path with the lowest accumulated cost, to each network in the internetwork.
OSPF has the following features:
Fast Convergence OSPF can detect and propagate topology changes faster than RIP. Count-to-infinity does not occur with OSPF.
Loop-Free Routes OSPF-calculated routes are always loop-free.
Scalability With OSPF, an AS can be subdivided into contiguous groups of networks called areas. Routes within areas can be summarized to minimize route table entries. Areas can be configured with a default route summarizing all routes outside the AS or outside the area. As a result, OSPF can scale to large and very large internetworks. In contrast, RIP for IP internetworks cannot be subdivided and no route summarization is done beyond the summarizing for all subnets of a network ID.
Subnet Mask Advertised with the Network OSPF was designed to advertise the subnet mask with the network. OSPF supports variable-length subnet masks (VLSM), disjointed subnets, and supernetting.
Support for Authentication Information exchanges between OSPF routes can be authenticated. Windows 2000 OSPF supports simple password authentication.
Support for External Routes Routes outside of the OSPF AS are advertised within the AS so that OSPF routers can calculate the least cost route to external networks.
Simple password authentication for OSPF is designed to prevent unauthorized OSPF routers from being placed on the network. The simple password is not secure, however, because it is sent on the network in clear text. Anyone with a protocol analyzer such as Microsoft Network Monitor can capture the OSPF messages and view the authentication password.