Calculate routing table memory footprint from route count and kernel FIB entry size.
The Linux kernel's IPv4 routing table (FIB) uses an LC-trie (level-compressed trie) structure that supports lookup in roughly O(log n) time relative to the number of routes — a significant improvement over a naive linear scan, which is what makes full-Internet-scale routing tables (800,000+ IPv4 routes) practically searchable in nanoseconds per lookup. Memory usage scales roughly linearly with route count, with per-route overhead depending on kernel version and how many route attributes (metrics, next-hops, source-specific routing) each entry carries.
Table memory and lookup complexity
memory ≈ routes × bytes_per_route; lookup_time ∝ log2(routes)
`ip route show table all | wc -l` counts entries across all tables; `wc -l < /proc/net/route` gives the IPv4 main table count directly from the kernel's exposed route file.
An LC-trie (level-compressed trie) is a compact prefix-tree structure optimized for longest-prefix-match lookups, which is exactly the operation IP routing requires (finding the most specific matching route for a destination) — it offers good average-case lookup time and reasonable memory usage even with hundreds of thousands of routes.
Yes — route table changes (additions/deletions/updates from routing protocols like BGP or OSPF) trigger FIB rebuilds or incremental updates, and very large or frequently-changing tables can increase CPU load during convergence events, which matters for routers handling full Internet routing tables.