Intermediate called intermediate systems (ISs) and other user devices

Intermediate
System to Intermediate System is an extensible intra domain routing protocol
designed by Digital Equipment Corporation (DEC) as part of DECnet Phase V
networks. IS–IS was made a standard routing protocol by the ISO in 1992 for
communication between network devices referred to as intermediate systems (Kaur
et al, 2014). The purpose of standardizing IS–IS is to make it possible for
packets to be routed in the OSI protocol suite that uses the
connectionless–mode network protocol (CLNP) and the connectionless–mode network
service (CLNS) to provide connectionless data delivery for the transport layer
within the protocol stack.

In
order to allow the CLNS to carry IP information, IS–IS was later extended to
support routing of data packets in IP, which has become the standard network
layer protocol for the internet. The IP implementation of IS–IS is called
integrated IS–IS. It was published in RFC 1195. The word integrated was used in
the sense that the protocol can be used to support network traffic in IP
environments only, OSI environments only, and can also support interconnection
between hosts in both environments.

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In
IS–IS networks, routers are called intermediate systems (ISs) and other user
devices are called end systems (ESs). The end systems and the intermediate
systems are grouped together to form a routing domain.

Similar to OSPFv3, Integrated IS–IS also uses
Dijkstra’s (SPF) algorithm to determine the shortest path to a destination in a
network. Each IS–IS router separately builds a topology database of the network
using link–state information collected from other routers in the network. Every
router in the routing domain sends an IS–IS Protocol Data Unit (PDU) or a
packet called Link State Packet (LSP), which contains information about itself
and the links attached to it. The LSP contains information encoded in a
variable length data structure that is made up of type, length, and value. This
data structure is often referred to as TLV (Hopps,