If you’re new to IS-IS you might have come across different people, posts and or different documentation referring to it as IS-IS or Integrated IS-IS among others. This post very briefly attempts to introduce and clarify why that might be the case if you ever wondered.
Initially standardised (ISO 10589) by the International Standards Organization (ISO), the group behind the OSI reference model we all learned to love, IS-IS was designed as a link-state dynamic routing protocol for routing CLNP traffic in the OSI CLNS environment. CLNP can be seen as approximate to IP in the TCP/IP reference model .
As a quick side note, a lot of people at times tend to confuse CLNP and CLNS believing them to be the same thing. Well, they’re not. CLNP is a network layer protocol whereas CLNS is a data communication service. I’ll write another post trying to explain my understanding of the differences between the two.
Since ‘no one size fits all’ also applies to networking, IS-IS was designed in such way which allows extensions to be added over time to support other network protocols making it a very extensible routing protocol. The Internet elders or wizards at the Internet Engineering Task Force (IETF) took full advantage of it and not too long after added support to its beloved IP protocol as they felt at the time there was a ‘critical need for routers to support both IP traffic and OSI traffic in parallel’ . At the time TCP/IP was still growing in popularity and it was expected its existence along side the OSI to continue for an extended period of time.
For the above reason–its support to multiple network layer routing protocols– IS-IS is referred to as Integrated IS-IS or Dual IS-IS. In other words, Integrated IS-IS is based on the OSI IS-IS routing protocol with IP-specific functions added. Chances are, if you hear people talking about IS-IS in your work environment more likely they’re talking about Integrated IS-IS. However, as you can appreciate Integrated IS-IS is a mouth full. I also use IS-IS whilst referring to Integrated IS-IS in occasions where the context is clear–and so does numerous Cisco literature. Having said that, just remember “long days and pleasant nights.”