System information
Troubleshooting ISO CLNS 12-273
ISO CLNS Addressing
Figure 12-2 IS-IS NSAP Addressing Structure
An IS-IS NSAP address is divided into two parts: an area address (AA) and a system ID. Level 2
routing uses the AA. Level 1 routing uses the system ID address. The NSAP address is laid out as
follows:
• The n-selector (S) is the last byte of the NSAP address.
• The system ID is found between the area address and the n-selector byte.
• The area address is the NSAP address, not including the system ID and n-selector.
The IS-IS routing protocol interprets the bytes from the AFI up to (but not including) the system ID
field in the DSP as an area identifier. The system ID specifies the system.
Addressing Rules
All NSAP addresses must obey the following constraints:
• No two nodes can have addresses with the same NET; that is, addresses can match all but the
n-selector (S) field in the DSP.
• ISO-IGRP requires at least 10 bytes of length; 1 for domain, 2 for area, 6 for system ID, and 1
for n-selector.
• Cisco’s implementation of IS-IS requires at least 8 bytes; 1 for area, 6 for system ID, and 1 for
n-selector.
• No two nodes residing within the same area can have addresses in which the system ID fields are
the same.
The following are examples of OSI network and GOSIP NSAP addresses using the ISO-IGRP
implementation. The second example is the OSI network NSAP address format:
47.0004.004D.0003.0000.0C00.62E6.00
|
Domain
|
Area
|
System ID
|
S
|
Entering Routes
Routes are entered by specifying pairs (NSAP prefix and next-hop NET). NETs are similar in
function to NSAP addresses. In the routing table, the best match means the longest NSAP prefix
entry that matches the beginning of the destination NSAP address. In Table 12-1, which is an
example of a static routing table, the next-hop NETs are listed for completeness but are not necessary
to understand the routing algorithm. Table 12-2 offers examples of how the longest matching NSAP
prefix can be matched with routing table entries in Table 12-1.
Table 12-1 Sample Routing Table Entries
Entry NSAP Address Prefix Next-Hop NET
1
47.0005.000c.0001 47.0005.000c.0001.0000.1234.00
IDP DSP
AFI IDI HO-DSP System ID S
1 Variable
IS-IS area address System address
61