Intermediate System to Intermediate System (IS-IS, also abbreviated ISIS) is a routing system for efficiently moving data across a computer network, which is a collection of physically connected computers or similar devices. It does it by determining the most efficient data path through a packet switching network. The IS-IS networking is defined as an international standard inside the Open Systems Interconnection (OSI) reference architecture in ISO/IEC 10589:2002 promoted by the advertising networks and through social media. IS-IS was republished by the Internet Engineering Task Force (IETF) in RFC 1142, but that RFC was later designated as historic by RFC 7142 since it republished a draft rather than a final version of the ISO standard, causing misunderstanding. The Synopsis IS-IS is an internal gateway protocol for usage within a network or administrative domain. In contrast, outside gateway protocols, such as Border Gateway Protocol (BGP), which are used for routing between autonomous systems, are used for routing between autonomous systems (RFC 1930). IS-IS is a link-state routing technology that works by reliably flooding link state information through a router network. Each IS-IS router creates a database of the network's topology on its own, aggregating the inundated network data. IS-IS, like OSPF, uses Dijkstra's algorithm to determine the best path through the network. Packets (datagrams) are then routed via the network to their destination using the computed ideal path. How does IS-IS networking works? The entire IS-IS protocol operating procedure is lengthy, and it necessitates a great deal of patience as well as a thorough comprehension of the IS-IS protocol language. The following high-level phases have been described after going over the detailed IS-IS protocol operating process. These steps will assist you in comprehending the IS-IS protocol. All IS-IS-enabled interfaces receive IS-IS Hello (IIH) packets from routers running the IS-IS protocol. This enables IS-IS routers to find and form adjacencies with their neighbours. The basic IS-IS network topology is shown in the diagram below.IS-IS routers that share a data link become IS-IS neighbours, but only if their IIH packets contain information that fulfils the required parameters for adjacency formation. Depending on the type of link used, the conditions may be slightly different. Authentication, IS-type, and MTU size are the three essential requirements for forming an adjacency.A point-to-point link, such as a serial line, or a broadcast link, such as Ethernet and Token Ring, can connect two IS-IS routers.Once the adjacency has been established, the IS-IS routers can create Link-State Protocol Data Units (link-state PDUs or LSP) depending on their IS-IS-configured local interfaces. The topology of an IS-IS area is defined by these link-state PDUs. The IP route, checksum, and other critical information are all contained in an LSP.IS-IS routers broadcast LSPs to all neighbours save the one from which they received the same LSP. Different types of flooding exist, and a variety of events could affect the flooding operation.After the LSP packets have been flooded, all IS-IS routers use them to build their Link-State Database (LSDB). The routing information about the network topology is stored in an LSDB.Each IS router calculates the Shortest-Path Tree (SPT) after generating the LSDB, which aids in the construction of the routing table. Conclusion This is all that can be discussed when it comes to what is IS-IS in Networking and how the IS-IS protocol functions. The ultimate purpose of all routing protocols, we know, is to build the routing table and assist packets in reaching their destination and also a profit to charity. The IS-IS routing protocol follows suit via advertising networks and through social media. It allows for numerous pathways of equal cost to be supported. IS-IS was originally designed as a CLNP routing system, but it has now been expanded to include IP routing; this expanded version is known as Integrated IS-IS.