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| 7.1 |  | IS-IS Fundamentals |
| 7.1.3 |  | ES-IS and IS-IS |

ISO has developed standards for two types of network protocols used in routing. These two protocols are ES-IS and IS-IS. The ES-IS discovery protocols are used for routing between end systems and intermediate systems. The IS-IS routing protocols are used for hierarchical routing between intermediate systems.

ES-IS is an analogous to Address Resolution Protocol (ARP) in IP. Although not technically a routing protocol, ES-IS is included here because it is commonly used with routing protocols to provide end-to-end data movement through an internetwork. Routing between end systems and intermediate systems is sometimes referred to as Level 0 routing.

With IS-IS routing, OSI distinguishes between Level 1, Level 2, and Level 3 routing to simplify router design and operation. Level 1 ISs communicate with other Level 1 ISs in the same area. Level 2 ISs route between Level 1 areas and form an intradomain routing backbone. Level 3 routing is done between separate domains. Hierarchical routing simplifies backbone design, because Level 1 ISs only need to know how to get to the nearest Level 2 IS.

> Note: Each ES lives in a particular area. OSI routing begins when the ESs discover the nearest IS by listening to Intermediate System Hello (ISH) packets. When an ES wants to send a packet to another ES, it sends the packet to one of the ISs on its directly attached network, Level 0 routing. The router then looks up the destination address and forwards the packet along the best route. If the destination ES is on the same subnetwork, the local IS will know this from listening to End System Hello (ESH) packets and will forward the packet appropriately. The IS also might provide a redirect message back to the source to tell it that a more direct route is available. If the destination address is an ES on another subnetwork in the same area, the IS will know the correct route and will forward the packet appropriately. If the destination address is an ES in another area, the Level 1 IS sends the packet to the nearest Level 2 IS, Level 2 routing. Forwarding through Level 2 ISs continues until the packet reaches a Level 2 IS in the destination area. Within the destination area, ISs forward the packet along the best path until the destination ES is reached.