
OSI Overview
117370-A Rev. A 1-13
Figure 1-7. NSAP Area Address
When a router receives a packet, it examines the contents of the packet’s NSAP
destination area address fields. The router compares its own NSAP area
address(es) with the NSAP destination address contained in the packet’s header. If
they match, then the destination system is in that router’s area. If the addresses do
not match, then the destination system is located in a different area and the router
must route the packet outside of the local area, using L2 routing services.
Allocating NSAP Addresses
To demonstrate how NSAP addresses are allocated, F
igure 1-8 shows a sample
OSI network set up on a college campus in the United States. To obtain and
allocate NSAP addresses for the OSI network, the network administrator did the
following:
1. Divided the campus OSI network into areas
The administrator divided the campus OSI network into Areas A, B, and C.
These three areas make up the campus routing domain.
DFIIDIAFI
AAI
or
ORG
Rsvd RDI Area ID S
Area Address
OSI0008A
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DFI Domain Format Identifier
AAI Administrative Authority Identifier
Rsvd Reserved
RDI Routing Domain Identifier
Area Area Identifier
ID System Identifier
S NSAP Selector
IDP Initial Domain Part
DSP Domain Specific Part
Key
ORG Organization Identifier
DSPIDP
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