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JunosE 11.3.x IP, IPv6, and IGP Configuration Guide
ISO Network Layer Addresses
Dynamic Hostname Resolution
320
ISO network layer addresses are flexible enough to make routing feasible in a worldwide
Internet. Network layer addresses in ISO and IP are hierarchical and clearly identify level
1 and level 2 areas. These addresses can be up to 20 octets long; any packet that contains
an address has one additional octet to specify the length of the address.
An ISO address—also known as the NSAP address—is broken into three parts: the area
address, the system identifier (ID), and the NSAP selector.
area address
system ID
selector
area address
system ID
The area address defines the routing domain and the area within the routing domain.
The length of the ID field can be from 1 to 8 octets and uses a single fixed length for any
one routing domain. The selector field is always 1 octet long. Usually, all end systems
within the same area have the same area address. Some areas can have multiple
addresses. The NSAP address is defined by the network entity title (NET) during
configuration.
Level 1 Routing
A level 1 router looks at a packet's area address and compares it with a destination
address. If the area portion of the destination address matches its own area's address,
the level 1 router uses the ID portion of the address to route the packet. If the area portion
of the address does not match, the level 1 router routes the packet to a level 2 router
within its area.
Level 2 Routing
Level 2 routers do not look at an area's internal structure, but simply route toward an
area based on the area address. It is common for a level 2 router to also be a level 1 router
in a particular area; these routers are sometimes referred to as level 1-2 routers. See
Figure 18 on page 319.
The system identifier of the NSAP address identifies a node in a network. System operators
often find symbolic hostnames to be easier to use and remember than the system
identifier. However, a static mapping of hostname to system identifier requires every
router to maintain a table of the mappings; each table must contain the hostnames and
system identifiers of every router in the network. The static mapping must be managed
by router operators, and every change or addition of a mapping requires all the tables to
be updated. Consequently, the static tables are likely to become rapidly outdated.
The router supports dynamic resolution of hostnames to system identifiers. You can use
the clns host command to map the hostname to the NSAP address, and therefore to
selector
Copyright © 2010, Juniper Networks, Inc.
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