Routing:

Routing is a major component of the network layer and is concerned with the problem of determining feasible paths (or routes) for packets to follow from each source to each destination. The best path is the one that gives minimum end-to-end delay and with the greatest available bandwidth.

Routing algorithms should seek one or more of the following goals:

1. Rapid and accurate delivery of packets.
2. Adaptability to changes in the network topology resulting from node or link failures.
3. Adaptability to varying source-destination traffic loads.
4. Ability to route packets away from temporarily congested links.
5. Ability to determine the connectivity of the network.
6. Ability to avoid routing loops.
7. Low overhead.

The routing algorithm is that part of the network layer software responsible for deciding which output line an incoming packet should be transmitted on.

OSPF:

The Open Shortest Path First or OSPF protocol is an intradomain routing protocol based on link state routing. Its domain is also an autonomous system.

Areas: To handle routing efficiently and in a timely manner, OSPF divides an autonomous system into areas. An area is a collection of networks, hosts, and routers all contained within an autonomous system. An autonomous system can be divided into many different areas. All networks inside an area must be connected.

Routers inside an area flood the area with routing information. At the border of an area, special routers called area border routers summarize the information about the area and send it to other areas. Among the areas inside an autonomous system is a special area called the backbone; all the areas inside an autonomous system must be connected to the backbone. In other words, the backbone serves as a primary area and the other areas as secondary areas. This does not mean that the routers within areas cannot be connected to each other, however. The routers inside the backbone are called the backbone routers. Note that a backbone router can also be an area border router.

If, because of some problem, the connectivity between a backbone and an area is broken, a virtual link between routers must be created by an administrator to allow continuity of the functions of the backbone as the primary area.

Each area has area identification. The area identification of the backbone is zero. Figure below shows an autonomous system and its areas.

Metric: The OSPF protocol allows the administrator to assign a cost, called the metric, to each route. The metric can be based on a type of service (minimum delay, maximum throughput, and so on).

Types of Links In OSPF terminology, a connection is called a link.

A point-to-point link connects two routers without any other host or router in between. In other words, the purpose of the link (network) is just to connect the two routers. An example of this type of link is two routers connected by a telephone line or a T line. There is no need to assign a network address to this type of link.

A transient link is a network with several routers attached to it. The data can enter through any of the routers and leave through any router. All LANs and some WANs with two or more routers are of this type.

A stub link is a network that is connected to only one router. The data packets enter the network through this single router and leave the network through this same router. This is a special case of the transient network

When the link between two routers is broken, the administration may create a virtual link between them, using a longer path that probably goes through several routers.