If the destination ES is on the same subnetwork, the local IS will know this from listening to ESHs, and will forward the packet appropriately. 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. 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. Each IS generates an update specifying the ESs and ISs to which it is connected, as well as the associated metrics.
Sequence numbers terminate the flood and distinguish old updates from new ones.
Because all ISs receive link state updates from all other ISs, each IS can build a complete full topology database. When the topology changes, new updates are sent. Metrics IS-IS uses a single required default metric with a maximum path value of 1,024. The metric is arbitrary and is typically assigned by a network administrator. Any single link can have a maximum value of 64.