Computer network protocol -OSPF

1 Introduction

• Open Shortest Path First OSPF (Open Shortest Path First) is to overcome the disadvantages of RIP developed in 1989, OSPF principle is very simple, but the implementation was more complex
• "Open" indicates OSPF protocol is not controlled by a single vendor, but published
• "Shortest Path First" due to the use of SPF Dijkstra shortest path algorithm proposed uses a distributed link state protocol (link state protocol)

Three points 2 OSPF protocol

Updating the routing table to rely on routers exchange information with each

1. And who exchange : this autonomous system in all routers exchange (Note: only in the RIP and in neighboring routers exchange)
2. What exchange : Link adjacent designated router, the router and which neighboring routers, and router link adjacent to "measure" (metric)

Measurement :

• It represents the cost, distance, bandwidth, delay, etc., which is determined by the network administrator, many of them with flexible
• The number of measures in the RIP protocol, is the "hops" between network is separated by the router

3. When the exchange : only when a change in link state exchange (Note: RIP protocol is a timing exchange)

3 link-state routing protocol

1. FIG have A, B, C, D are respectively connected to the router via four different link, they will send the link-state router adjacent to all other routers in the autonomous system by the OSPF protocol, then each router database link on the preservation of the status of all routers in the autonomous system, that is to say four routers are preserved three other link-state router
2. The link-state database in each router, can be obtained with weights of autonomous system digraph
3. In weighted directed graph based, themselves as the root node, to calculate the shortest path to the other nodes, such as node A, node A acquires the shortest path to the respective nodes, the node A is obtained a routing table, such as FIG to reach router D, then to C through the router, the router B to reach directly up to the next-hop router B

The concept of area 4

• OSPF can be used to make very large network, OSPF will be an autonomous system is subdivided into a number of smaller ranges, called region
• Has a 32-bit identifier for each region, in dotted decimal notation, in the following figure 0.0.0.1to a region identifier, the blue part of the entire autonomous system, and each part is divided regions green
  
• The area is not too large, the best router in an area not more than 200
• May be used in the interior region of the OSPF protocol, which is the shortest path to generate the original OSPF protocol used in FIG entire autonomous system, in each area into the shortest path generating FIG, each area not just its own internal routing link status broadcast out , thus reducing the overall network traffic, to improve the efficiency of network communication