IS-IS is a Link State dynamic routing protocol that uses cost as its cost function to determine best paths for traffic.
IS-IS Metric - Cost
Unlike Open Shortest Path First (OSPF), IS-IS does not automatically derive the cost from interface bandwidth - it assumes that all links have an equal cost by default. Thus, if we do not manually set link costs, it will take the path with the lowest hope count (like Routing Information Protocol (RIP)).
Assuming 100 Mbps accross the top, and 10 Mbps via R5.
In this scenario, IS-IS will prefer the slow route, via R5, unless we manually input the cost of different links.
Why IS-IS?
IS-IS links need to be manually configured, or it will default to naive hop count. However, it does provide for very large scalability.
As a result, it is typically only used in Service Provider networks or large organizations with their own MPLS network.