Discussion

Chapter 11
CHAPTER 11
IS-IS11
11.0 Introduction
The Intermediate System-to-Intermediate System (IS-IS) protocol is an IGP that
routes packets within a single autonomous system (AS), or domain. IS-IS is based on
the DECNET Phase V network technology, which was developed at Digital Equipment
Corporation (DEC) in the 1980s and was initially standardized by ANSI as the
International Organization for Standardization (ISO) intradomain protocol in ISO/IEC
10589. The first version of IS-IS was designed to work on the OSI Connectionless Network
Protocol (CNLP). RFC 1195, published in 1990, added extensions to support
IP routes.
As an IGP, IS-IS works within a routing domain, which usually corresponds to an
administrative boundary, and focuses on determining the most efficient routes to
destinations within a domain. This is in contrast with EGPs, whose primary focus is
on policy rather than on the most efficient routing. An IS-IS routing domain consists
of end systems, which send and receive packets, and intermediate systems (the ISO
term for a router), which receive and forward packets.
IS-IS is a link-state protocol and uses link-state protocol data units (link-state PDUs,
or LSPs) to describe the network topology. Each IS-IS router generates LSPs that
describe the topology, along with IP routes, checksums, and other information, and
floods the LSPs throughout the domain. Each router ends up with a link-state database
that describes the same network topology. Once the router has the complete
network topology, it runs the Dijkstra shortest-path first (SPF) calculation to determine
the shortest path to each destination in the network. The calculation results in
destination/next-hop pairs that are placed in the IS-IS routing database. Each router
performs the SPF calculation independently, and each IS-IS router has an identical
database as a result.
Unlike other IP routing protocols, IS-IS runs directly on the data link layer (Layer 2
of the OSI model) and does not need addresses on each interface, just on the router
itself. This makes IS-IS configuration simpler.
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