6.4. Integrated IS-IS
You will often find that books written about IS-IS are written as a comparison to OSPF. This happened over time because OSPF was typically chosen as a more widely supported native IP protocol. Currently, it is more difficult to find information and expertise on IS-IS than on OSPF. With that being said, many of the largest internetworks in place today use IS-IS.
IS-IS is most commonly used by ISPs. IS-IS makes a great routing protocol for large internetworks because it is simple and stable. IS-IS is the primary protocol for use in networks that support Open Systems Interconnection (OSI) protocols.
If you go back and look at the history of link state routing protocols, the development of IS-IS actually started before OSPF. ...