IS-IS Routing Protocol
Addressing and levels
IS-IS runs directly on the link layer and identifies routers with a Network Entity Title (NET), an NSAP whose NSEL is 00. The NET combines an area ID and a 6-byte system ID, for example 49.0001.1921.6800.1001.00: area 49.0001, system ID 1921.6800.1001. Level 1 is intra-area; Level 2 is the backbone. A Level 1-2 router advertises a default toward Level 1 by setting the Attached (ATT) bit in its Level 1 LSP, but only when it has a usable Level 2 adjacency. Nokia SR OS configures this under router isis 0 with area-id, system-id, and level-capability level-1|level-2|level-1-2.
Adjacency and flooding
On broadcast links, IS-IS elects a Designated Intermediate System (DIS): highest interface priority, then highest MAC (SNPA). There is no backup DIS and election preempts. The DIS originates a pseudonode LSP and sends CSNPs every 10 s; other routers send PSNPs. On point-to-point links no DIS exists, so set interface-type point-to-point on Nokia interfaces to avoid DIS and pseudonode overhead. LSPs flood within a level; mesh-group controls flooding on highly meshed NBMA links.
Metrics and path choice
Narrow metrics are 6-bit (1-63); wide metrics are 24-bit and support traffic engineering. Nokia SR OS can be restricted with wide-metrics-only. IS-IS adds interface metrics along the path; it does not use bandwidth by default.
Worked example
R1 priority 64, MAC ...0a; R2 priority 64, MAC ...1f; R3 priority 100, MAC ...03; R4 priority 0 on one LAN. DIS is R3: priority 100 beats 64. If R3 fails, R2 wins: R1 and R2 tie on priority, so the higher MAC ...1f wins. R4 never wins with priority 0. If the design needs R1 as DIS, raise R1 priority above 100; do not rely on a reboot.
Traps
Area mismatch blocks Level 1 adjacency but not Level 2. Level mismatch blocks both. passive advertises the prefix without Hellos. The overload bit still allows traffic to the router's own prefixes. ATT is not set without a Level 2 adjacency.