IS-IS: Levels, Addressing, Metrics, and Convergence
Addressing
IS-IS runs directly over CLNS, so it can route before IP is configured. Every router needs a unique NET in the form 49.0001.1921.6800.1001.00: the leading bytes are the area ID, the middle 6 bytes the system ID, and the trailing 00 the NSEL (always 00). Two routers form an L1 adjacency only if their area IDs match; L2 adjacencies ignore the area ID.
Levels and multi-area behavior
- L1 routers keep a single-area database and reach the rest of the domain through a default route.
- L2 forms the backbone between areas.
- L1/L2 routers are the inter-area points; they set the attached (ATT) bit in their L1 LSP, which is how L1 routers learn to point a default toward the nearest L2-capable router.
Trap: leaking L2 prefixes down into L1 is not automatic. You must write an export policy that advertises those routes into IS-IS. Without it, L1 routers see only a default.
Metrics
IS-IS metrics are per-interface and add along the path. Narrow metrics are 6-bit (1–63 per link, 1023 total); wide metrics are 24-bit. Change metric style across an entire level at once - a mismatch breaks SPF computation even though the adjacency may still show up.
Broadcast behavior and convergence
On broadcast media a DIS is elected per level by highest priority, then highest MAC. Unlike OSPF's DR there is no backup, and the DIS is preemptive - a new higher-priority router takes over immediately. The DIS floods CSNPs every 10 seconds. Default hellos are 10 s with a 3× hold multiplier; BFD, LSP/SPF throttling and graceful restart shorten outages. Authentication (hello/SNP/LSP, HMAC-MD5 key chains) must match on both sides.
Rule of thumb
Same area ID for L1, unique system IDs everywhere, wide metrics on all routers in the level.