IS-IS
acronymnetworking
Stands for: Intermediate System to Intermediate System
Intermediate System to Intermediate System - OSPF's link-state twin from the OSI world: the protocol you have probably never configured, running the carrier backbones your traffic crosses every day.
IS-IS (ISO 10589, extended for IP by RFC 1195) plays the same game as OSPF - flood the map, run Dijkstra - with a different pedigree and two habits that aged remarkably well. It runs directly over layer 2, not over IP, so it cannot be attacked or broken by the very routing it computes; and everything it carries is a TLV, so extending it for IPv6, traffic engineering, or Segment Routing meant new TLVs rather than new protocol versions. Its two-level hierarchy (level 1 areas, level 2 backbone) maps loosely onto OSPF's areas-and-area-0. Large ISPs standardized on it decades ago and never left; enterprises mostly meet it inside data-center fabric underlays.
IS-IS and OSPF solve the same problem with similar mathematics and different politics. Both are link-state protocols where every router builds an identical map of the area and runs Dijkstra against it. The difference that matters operationally is that IS-IS does not run over IP at all: it rides directly on layer two, which means the control plane cannot be reached by IP traffic and therefore cannot be attacked from it.
That property, plus a design that treats new address families as extensions rather than rewrites, is why IS-IS is disproportionately common in large service provider networks. Adding IPv6 required a new type of information element rather than a parallel protocol instance, which is the sort of detail that decides architecture at scale. Enterprises more often run OSPF, largely because vendor documentation, training, and hiring pools all lean that way.
The vocabulary is the main obstacle for engineers coming from OSPF. Areas become levels, routers become intermediate systems, and the addressing is NSAP rather than anything familiar. Underneath the unfamiliar words the concepts map closely, and the practical advice is to learn the translation once rather than to treat IS-IS as a different discipline.
Also known as: is-is, isis, clns, net address, level 1, level 2