So in this section we're going to take a look at OSPF. OSPF is the Open Shortest Path First routing protocol. Open means it's an Open Standard. It doesn't mean opening, as in "opening and shutting" as a lot of people think. It actually means an Open Standard, developed by the IETF, by the OSPF Working Group for IP networks, and first documented in RFC1247. The current version of the standard, is OSPFv2 documented in RFC2328. The Shortest Path First in the name, refers to Edsger Dijkstra's algorithm for producing the shortest path tree through a graph. And you can read all about that in the reference below, the paper that Dijkstra produced in 1959. OSPF is known as a Link State Routing Protocol. The other Link State Routing Protocol is IS-IS. Where each node in the network computes the map of connectivity through the network. The other type of Routing Protocol is known as Distance Vector. Examples of Distance Vector protocols include Cisco's EIGRP and RIP. Each node here, shares its view of the routing table with other nodes. Routers with OSPF enabled on them, look for neighboring routers also running OSPF. To do this we use what's known as the "Hello" protocol. The "Hello" packet includes the subnet mask, list of known neighbors, and details such as the "hello interval" and the "router dead interval". The "hello interval" is how often the router will send Hellos, and the "router dead interval" is how long to wait before deciding a router has disappeared. The values of "hello interval", "router dead interval", and subnet mask must match on both neighbors before an adjacency is established. When a neighboring router responds with matching details, a neighbor relationship is formed. A relationship is formed between selected neighboring routers for the purpose of exchanging routing information. And this is what we call an "Adjacency". Not every pair of neighboring routers become adjacent. On multi-access networks, for example Ethernet, only selected routers will form adjacencies. Once the adjacency is formed, neighbors share the link state information. Information goes into a Link State Packet or LSP. LSPs are sent to a neighbor and these are known as Link State Announcements or LSAs. New information received from neighbors is used to compute a new view of the network. New LSPs are flooded and the routers recompute the routing table. All routers across the network will form neighbor relationships with their directly attached neighbors. Each router computes the routing table. Once each router has the same view of the network, the network has converged. The IGP design for a network is crucially important to ensure scalability and rapid convergence. The general rule: the fewer the prefixes, the faster the convergence. And network operators work really hard to make sure they minimize the number of prefixes carried in OSPF, to ensure the fastest possible convergence of the protocol.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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