IS-IS
Addresses and levels
IS-IS runs directly over Layer 2, so it needs no IP address to form an adjacency; identity comes from the NET: a 1-to-13-byte area, a 6-byte system ID, and the NSEL 00, for example 49.0001.1921.6800.1001.00. The system ID must be unique (usually derived from the loopback) and changing it breaks existing adjacencies. Level 1 builds a topology inside one area, Level 2 forms the backbone between areas, and an L1/L2 router sets the attached bit so L1 routers can install a default route toward the nearest L2 router.
DIS and metrics
Each broadcast circuit elects a Designated IS: highest priority (default 64), then highest MAC/SNPA. Unlike OSPF's DR, the DIS is preemptive and has no backup. Metrics are either narrow (6-bit, default value 10, maximum 63) or wide (24-bit, configured with wide-metrics-only). Both ends of an adjacency must agree on the style or you get metric mismatches. Metrics are per interface and additive.
PDUs and traps
IIH forms adjacencies, LSPs carry prefixes and reachability, and CSNP/PSNP synchronize databases. Frequent question traps: forgetting the passive loopback (set protocols isis interface lo0.0 passive), mixing narrow and wide metrics across a link, expecting automatic summarization between levels (L1 routes are not leaked into L2 by default; you need route leaking), and misunderstanding the overload bit, which tells other routers not to transit traffic through this device during maintenance.
Worked example
Three routers in a chain with the default narrow metric of 10 per link: the far end is two hops away and therefore costs 20. If a link is upgraded to wide metrics, remember to set the same style on every interface in that level, or the adjacency will disagree on metric values.