Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 4 IS-IS Configuration
4-2
4.1.1 Basic Concepts
I. IS-IS terminology
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Intermediate system (IS). An IS, similar to a router in TCP/IP, is the basic unit in
IS-IS protocol to generate and propagate routing information. In the following text,
an IS is a router.
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End system (ES). An ES refers to a host system in TCP/IP. ISO defines the ES-IS
protocol for communication between an ES and an IS, therefore an ES does not
participate in the IS-IS process.
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Routing domain (RD). A group of ISs exchange routing information with the same
routing protocol in a routing domain.
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Area. An area is a division unit in a routing domain. The IS-IS protocol allows a
routing domain to be divided into multiple areas.
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Link State Database (LSDB). All link states in the network forms the LSDB. There
is at least one LSDB in each IS. The IS uses SPF algorithm and LSDB to generate
its own routes.
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Link State Protocol Data Unit (LSPDU) or Link State Packet (LSP). Each IS can
generate a LSP which contains all the link state information of the IS.
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Network Protocol Data Unit (NPDU). An NPDU is a network layer protocol packet
in ISO, which is equivalent to an IP packet in TCP/IP.
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Designated IS. On a broadcast network, the designated router is also known as
the designated IS or a pseudonode.
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Network service access point (NSAP). The NSAP is the ISO network layer
address. It identifies an abstract network service access point and describes the
network address in the ISO reference model.
II. IS-IS address structure
1) NSAP
As shown in
, the NSAP address consists of the Initial Domain Part (IDP) and
the Domain Specific Part (DSP). The IDP is equal to the network ID of the IP address,
and the DSP is equal to the subnet and host IDs.
The IDP, defined by ISO, includes the Authority and Format Identifier (AFI) and the
Initial Domain Identifier (IDI).
The DSP includes the High Order DSP (HODSP), the System ID and SEL, where the
HODSP identifies the area, the System ID identifies the host, and the SEL indicates the
type of service.
The length of IDP and DSP is variable. The length of the NSAP address varies from 8
bytes to 20 bytes.
