TL&DR: Most probably not, but if you do, you’d better not rely on random blogs for professional advice #justSaying 😜

Here’s an interesting question I got from a reader in the midst of an OSPF-to-IS-IS migration:

Why should one bother with different [IS-IS] areas when the routing hierarchy is induced by the two levels and the appropriate IS-IS circuit types on the links between the routers?

Well, if you think you need a routing hierarchy, you’re bound to use IS-IS areas because that’s how the routing hierarchy is implemented in IS-IS. However…

• People were building networks with hundreds of routers in a single IS-IS area in the 90s.
• Because of that, the IS-IS code in major network operating systems tends to be pretty decent.
• As a consequence, unless you have a ginormous network, you’ll probably be just fine with a single IS-IS area. Just make sure all your routers are either L1-only or L2-only intermediate systems, or you’ll get an explosion in the LSP database size (more details).

On the other hand, you might be unfortunate enough to own inferior hardware that cannot cope with too many changes or large routing tables. In that case, the IS-IS routing hierarchy may still be useful for fault isolation¹ and route summarization.

However, keep in mind that IS-IS wasn’t designed to route IP². For example, by default, level-1 routers in an IS-IS area know only that there’s another router in their area attached to the level-2 backbone, and use that information to generate a default route. While you can inject level-2 routes into a level-1 area (and summarize prefixes in the process), this process must be configured and may not be supported on all platforms.

Speaking of platform support: there are IS-IS implementations out there (for example, FRRouting) that do not implement inter-level prefix propagation³. Checking the capabilities of your network devices before designing your IS-IS network is, unfortunately, still a thing in 2026.

Want to know more about how IS-IS areas really work? Dan Partelly created a beautiful set of (free) IS-IS lab exercises that walk you through the details:

• Multilevel IS-IS Deployments
• Distributing Level-2 IS-IS Routes into Level-1 Areas
• Summarizing Level-1 Routes into Level-2 Backbone

Even running these exercises is free: all you have to do is create a GitHub Codespaces instance, install the Arista cEOS container in it (or use SR Linux), and start kicking the tires.