What determines the preferred route to a destination in an IS-IS network?

IS-IS is a link-state protocol that, like OSPF, runs Dijkstra's Shortest Path First algorithm independently on each router against its own copy of the synchronized link-state database to compute the best path toward every reachable destination. Each link within the topology carries an administrator-assigned or default metric, and as the SPF algorithm walks the topology graph from the local router outward, it sums the metrics of every link traversed along a candidate path to produce that path's total accumulated cost. Among all possible paths to a given destination prefix, IS-IS always selects and installs the path (or, in the case of equal-cost multipath, the set of paths) with the numerically lowest total accumulated metric, exactly mirroring the 'lowest cost wins' principle shared by essentially all link-state IGPs. A higher accumulated metric value is by definition a less preferred, more costly path, making that option the direct inverse of correct IS-IS behavior. Level 2 proximity to the destination has no independent bearing on path selection beyond however it happens to be reflected in the accumulated metric itself — IS-IS does not apply a separate rule preferring paths merely for passing near a Level 2 router. Likewise, LSP recency (sequence number or freshness) governs which version of a given LSP is trusted and flooded during database synchronization, ensuring the topology database itself is accurate and loop-free, but it plays no role in the SPF cost comparison once the database is synchronized and consistent. Reference topics: Junos Enterprise Routing – IS-IS, SPF Calculation and Metric-Based Path Selection.