Flat/Single Level vs. Multi Level IS-IS Design Comparison. Flat routing means, without hierarchy, entire topology information of the network is known by each and every device in the network. IS-IS has two levels. Thus, for IS-IS, Multi Level means Two Level IS-IS. Level 1 and Level 2. When we have two levels, Level 1 routers don't know the topology of Level 2 and vice versa. By hiding topology information of different level routers, scalability is achieved.
Reason we achieve more scalable network is when there is a failure or new information added or metric changes in one Level, another level doesn't run SPF algorithm. But what are the design consideration when we have Flat or Multi Level IS-IS networks. Is Multi Level IS-IS design, which mean, Hierarchical IS-IS design always good? Answer is no. Although Multi Level provides Scalability, it comes with extra complexity and end to end routing convergence time increase.
So, I prepared below comparison charts to discuss different design aspects when it comes to IS-IS Single vs. Multi Level design. If you like this comparison chart, you can see more of them in my CCIE Enterprise Training.