Is-Is Overview - 3Com 7757 Configuration Manual

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IS-IS Overview

Basic Concept
IS-IS C
ONFIGURATION
The intermediate system-to-intermediate system (IS-IS) is a dynamic routing
protocol standardized by the International Organization for Standardization (ISO)
to operate on connectionless network protocol (CLNP).
The IS-IS routing protocol has been adopted in RFC 1195 by the International
Engineer Task Force (IETF) to be applied in both TCP/IP and OSI reference models,
and this form is called Integrated IS-IS or Dual IS-IS.
The IS-IS routing protocol, based on the link state algorithm, is an interior gateway
protocol (IGP) used within an Autonomous System. It is similar to open shortest
path first (OSPF) using shortest path first (SPF) algorithm to calculate best paths in
the network.
IS-IS terminology
Intermediate system (IS). An IS, similar to a router in TCP/IP, is the basic unit in
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IS-IS protocol to generate and propagate routing information. In the following
text, an IS equals to a router.
End system (ES). An ES refers to a host system in TCP/IP. ISO uses ES-IS protocol
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to specify the communication between an ES and an IS, therefore an ES does
not participate in the IS-IS process and can be ignored in the IS-IS protocol.
Routing domain (RD). A group of ISs exchange routing information with the
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same routing protocol in a routing domain.
Area. An area is a division unit in a routing domain. The IS-IS protocol allows a
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routing domain to be divided into multiple areas.
Link state database (LSDB). All link states in the network consist of the LSDB.
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There is at least one LSDB in each IS. The IS uses SPF algorithm and LSDB to
generate its own routes.
Link state protocol data unit or link state packet (LSP). In the IS-IS routing
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protocol, each IS can generate a LSP which contains all the link state
information of the IS. Each IS collects all the LSPs in the local area to generate
its own LSDB.
Network protocol data unit (NPDU). An NPDU is a network layer protocol
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packet in ISO, which is equivalent to an IP packet in TCP/IP.
Designated IS. On a broadcast network, the designated router is also known as
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the designated IS or a pseudonode.
Network service access point (NSAP). The NSAP is the ISO network layer
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address. It identifies an abstract network service access point and describes the
network address in the ISO reference model.