IS-IS Addressing
What the exam tests
Addressing is where many candidates lose easy marks: NSAP format, NET, system ID length, area address rules, and how Nokia SR OS derives or accepts these values.
Core rules
- An NSAP is the ISO address; a NET is an NSAP whose NSEL byte is 00.
- Structure: AFI (1 byte, 49 for private addressing), area ID (variable), system ID (6 bytes in common designs), NSEL (1 byte).
- The system ID must be unique within the routing domain and is usually 6 bytes; all routers should use the same system ID length.
- The area ID defines the Level 1 area; routers in the same L1 area must share at least one area address.
- A router may have multiple NETs for area migration, but they must share the same system ID.
- Maximum NET length is 20 bytes.
Worked example
Take 49.0001.1921.6800.1001.00. AFI is 49, area ID is 0001, system ID is 1921.6800.1001, NSEL is 00. If you change the area to 0002, the router forms L1 adjacencies only with routers that also have 0002 or an additional common area address. A second NET such as 49.0002.1921.6800.1001.00 allows migration while keeping the system ID stable.
Traps
Setting NSEL to anything other than 00; using different system ID lengths; duplicating a system ID; assuming the IS-IS area ID has the same meaning or format as an OSPF area.