Juniper IGP - CONFIGURATION GUIDE V11.1.X, Configuration Manual

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ISO Network Layer Addresses
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 selector
g016497
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 327.
Dynamic Hostname Resolution
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
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Overview
JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide