Iso Network Addresses - Juniper ACX1000 Configuration Manual

ISO Network Addresses

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(forward) packets. (Intermediate system is the Open System Interconnection [OSI] term
for a router.) ISO packets are called network PDUs.
In IS-IS, a single AS can be divided into smaller groups called areas. Routing between
areas is organized hierarchically, allowing a domain to be administratively divided into
smaller areas. This organization is accomplished by configuring Level 1 and Level 2
intermediate systems. Level 1 systems route within an area; when the destination is
outside an area, they route toward a Level 2 system. Level 2 intermediate systems route
between areas and toward other ASs. No IS-IS area functions strictly as a backbone.
Level 1 routers share intra-area routing information, and Level 2 routers share interarea
information about IP addresses available within each area. Uniquely, IS-IS routers can
act as both Level 1 and Level 2 routers, sharing intra-area routes with other Level 1 routers
and interarea routes with other Level 2 routers.
The propagation of link-state updates is determined by the level boundaries. All routers
within a level maintain a complete link-state database of all other routers in the same
level. Each router then uses the Dijkstra algorithm to determine the shortest path from
the local router to other routers in the link-state database.
IS-IS uses ISO network addresses. Each address identifies a point of connection to the
network, such as a router interface, and is called a network service access point (NSAP).
IS-IS supports multiple NSAP addresses on the loopback lo0 interface.
An end system can have multiple NSAP addresses, in which case the addresses differ
only by the last byte (called the n-selector). Each NSAP represents a service that is
available at that node. In addition to having multiple services, a single node can belong
to multiple areas.
Each network entity also has a special network address called a network entity title (NET).
Structurally, an NET is identical to an NSAP address but has an n-selector of 00. Most
end systems and intermediate systems have one NET. Intermediate systems that
participate in multiple areas can have multiple NETs.
The following ISO addresses illustrate the IS-IS address format:
49.0001.00a0.c96b.c490.00
49.0001.2081.9716.9018.00
NETs take several forms, depending on your network requirements. NET addresses are
hexadecimal and range from 8 octets to 20 octets in length. Generally, the format consists
of an authority and format Identifier (AFI), a domain ID, an area ID, a system identifier,
and a selector. The simplest format omits the domain ID and is 10 octets long. For
example, the NET address 49.0001.1921.6800.1001.00 consists of the following parts:
49—AFI
0001—Area ID
Chapter 18: Configuring Routing Protocols
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