Manual routes with ip route <net> <mask> <next-hop|interface> — four flavours worth knowing:

• Default route 0.0.0.0/0 — the gateway of last resort, used only when nothing more specific matches.
• Floating static: same prefix with a higher AD (e.g. ... 192.168.1.2 5) — sleeps until the primary route vanishes. Backup via AD, nothing else.
• Recursive: next-hop IP only; router resolves the outgoing interface through the RIB.
• Fully specified: interface + next-hop IP in one line, no extra lookup.