| Level 0 (L0) Routing | OSI routing begins with ES-IS, with the ESs discover the nearest IS by listening to ISH packets. When an ES needs to send a packet to another ES (either on the same or another area), it sends the packet to an IS on an attached network. This process is known as L0 routing. |
| Level 1 (L1) Routing (intra-area routing) | Every ES and IS resides in a particular area. In order to forward traffic, the router looks up the destination address and forwards the packet via the best route. If the destination is on the same subnet, the IS knows the location of the ES by listening to the ESHs, therefore able to forwards the packet appropriately. The IS can also send a redirect message back to the source ES to tell it that a direct route is available. If the destination is on a different subnet but within the same area, the IS identifies the best path using the System ID, and forwards the traffic appropriately. This process is known as L1 routing. |
| Level 2 (L2) Routing (inter-area routing) | If a destination address is in another area, the L1 IS sends the packet to the nearest L1/L2 IS. Packet forwarding continues through L1/L2 and L2 ISs using the area address, until the packet reaches an L1/L2 IS in the destination area. This process is known as L2 routing. Within the destination area, L1 ISs forward the packet along the best path using the System ID, until the packet reaches the destination. |
| Level 3 (L3) Routing (inter-domain routing) | Routing between separate IS-IS domains is called L3 routing. L3 routing is similar to the Border Gateway Protocol (BGP) inter-domain routing in TCP/IP. L3 routing passes traffic between different autonomous systems which have different routing logic; therefore metrics cannot be compared directly. L3 OSI routing is not implemented in Cisco IOS but is specified and accomplished through the Inter-Domain Routing Protocol (IDRP). Note: Cisco IOS does not support IDRP. |
OSI Routing Levels
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