Cisco IOS IS-IS Configuration: NET, Levels, and Backbone Routing
Working reference for IS-IS on Cisco IOS. What makes IS-IS different (runs on Layer 2 / CLNS not IP, doesn't need IP addresses to form adjacencies, carries multiple address families in one process). The NET address breakdown (AFI, area, system-id, SEL) and convention of deriving System ID from loopback. Levels: L1 intra-area, L2 backbone, L1/L2 ABR-equivalent routers, with circuit-type per-interface control. Basic configuration (net, is-type, ip router isis on interface), loopback inclusion for /32 propagation, default cost of 10 and metric-style wide for headroom, MD5 authentication. Verification (show isis neighbors / topology / database, show clns commands). IS-IS vs OSPF comparison. Pitfalls: wrong NET, circuit-type mismatch, narrow metric overflow, missing ip router isis, discontiguous L2 backbone.