Thus, ES-IS protocols allows ESs and ISs to identify each other. IS-IS provides routing between ISs. IS-IS periodically floods the network with link state information, allowing each router to maintain a complete picture of the network's topology. Optional path metrics available with IS-IS are based on delay, expense, and error. The delay metric represents the amount of delay on a particular link.
Expense relates to the communications costs of using the link.
The error metric represents the error rate of the link. Each IS-IS packet has a common header followed by an additional fixed portion and variable portion. The key to answering this question is to evaluate your internetworking requirements and then choose the best one. In general, RIP should satisfy the needs of smaller networks operating with low to moderate utilization. OSPF is a good all-around choice, especially for larger networks having requirements for varying service requests.