Routing in OSI uses hierarchies or levels depending on the scope in which routing is performed. There are four levels of routing in OSI:
- Level 0 – Routing between two ES nodes on the same link or between an ES node and its nearest IS
- Level 1 – Routing between ES nodes in a single area of a domain
- Level 2 – Routing between ES nodes in different areas of a domain
- Level 3 – Routing between ES nodes in different domains
Level 0 routing only deals with end host machines and other hosts on the same local network, or to their immediate gateway (router). This is accomplished by both ES and Is sending periodic Hello messages advertising their existence. Hellos sent by ES nodes are called ES Hellos (ESH) while Hellos sent by IS nodes are called IS Hellos (ISH). Level 0 routing is also referred to as ES-IS routing.
Level 1 routing is concerned with intra-area routing, or routing between ES nodes in the same area. An area is the administrative partitioning of a domain so it means the same. IS nodes in an area will have a detailed and complete visibility of the entire area’s topology. IS nodes collect lists of all ES nodes directly attached to them and advertise these lists to other IS nodes to learn the placement of all ES nodes.
Level 2 Routing is concerned with inter-area routing. Is nodes that contain all the addresses of ES nodes will advertise those to other IS nodes that exist in different areas. This allows ES nodes to reach other ES nodes that exist in other areas. ES nodes will forward packets to the closes IS if it is unaware of where a node is located, since IS nodes connected to each other contain the routes for all ES nodes allows this connectivity between ES nodes in different areas.
Level 3 routing is concerned with interdomain routing, this is similar to inter-autonomous system routing in TCP/IPs BGP routing protocol.