Before the explosion of routing tables and of forwarded traffic, the classic routing paradigm constituted routing table lookups in increasingly complex routing tables. These tables were populated by more or less scalable and fast-converging dynamic routing protocols together with connected routes and static routes.
This paradigm has changed rapidly with the innovations in switching technology (for example, Cisco Express Forwarding, or CEF, an example of fast switching). This new paradigm tries to switch traffic by tagging it or by facilitating other measures to avoid costly Layer 3 routing table lookups. "Costly" refers to keeping the CPU busy and clogging the bus instead of intra-ASIC or intra-linecard switching/forwarding. As discussed later, the signaling and forwarding is done differently within UNIX IP stacks. Signaling refers to exchanging protocol and reachability information, whereas forwarding actually moves packets, frames, or datagrams between gateway interfaces.