21.12 ROUTING WITHIN AUTONOMOUS SYSTEMS

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If the autonomous system consists of a small number of networks, the administrator can manually change the routing tables. If the system is very large and changes in topology (addition and deletion of networks) are frequent, an automatic procedure is required. Routing protocols used within an autonomous system are called interior gateway protocols (IGPs). A number of IGPs are used because the autonomous systems vary widely—it may be a small system with few LANs or a WAN with a large number of networks. Open shortest path first (OSPF) is the most efficient protocol used for routing within an autonomous system. In OSPF, routing is done based on the shortest path—the shortest path can be calculated between two nodes using criteria such as distance, dollar cost, and so on.

Routing protocols used within the autonomous system are called interior gateway protocols. Open shortest path first (OSPF) is one such protocol.

21.12.1 Open Shortest Path First (OSPF)

Interior router protocol (IRP) passes routing information between routers within an autonomous system. Open shortest path first (OSPF) protocol is used as the interior router protocol in TCP/IP networks. OSPF routing is a dynamic routing technique. As the name implies, the OSPF protocol computes a route through the Internet that incurs the least cost. It is analogous to a salesman finding the shortest path from one location to another location. The shortest path can be calculated based on the distance to be traveled or based on the cost of travel. Similarly, the cost of routing the packets can be based on a measure that can be configured by the administrator—it can be a function of delay, data rate, or simply dollar cost. Each router maintains the database of costs. Any change in costs is shared with other routers.

In OSPF protocol, the shortest path is calculated from the source to the destination, and routing is done using this path. The criteria for calculation of the shortest path can be distance or cost.

21.12.2 Flooding

Flooding is another protocol that can be used for routers to share the routing information. In flooding, each router sends its routing information to all the other routers to which it is directly connected. This information is shared again by these routers with their neighbors. This is a very simple and easy-to-implement protocol. However, the traffic on the network will be very high, particularly if the flooding is done very frequently.

In flooding, each router sends the packets to all the other routers connected to it. Routing information is shared among the routers using this protocol.

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