Forwarding vs Routing

Both are network-layer functions, but they belong to different planes and work at completely different timescales.

Aspect	Forwarding	Routing
Meaning	Move a packet from an input link interface to the correct output link interface	Determine the end-to-end path (route) a packet must take from source to destination
Scope	Local — decided by a single router	Global — decided collectively by all routers in the network
Plane	Data plane (packet forwarding)	Control plane (routing algorithms)
Timescale	Per packet — nanoseconds to microseconds	Per connection / periodically — milliseconds to minutes
Inputs	Destination IP address + forwarding table	Topology, link costs, metrics, policies, traffic load
Mechanism	Match (address lookup) + action (switch to output port), using ASICs / switching fabric / TCAM	Routing protocols exchange information (DV, LS, path vector) and compute paths
Table used	Forwarding table: dest prefix → output port	Routing table / RIB from which the forwarding table is derived
Examples	IP longest-prefix match	OSPF, RIP, BGP
Analogy	The signpost at a junction telling you which exit to take	Planning the whole trip on the map before leaving

Relationship between the two

Routing computes the paths; forwarding uses them. The routing algorithms run in the control plane, place the resulting routes in each router's routing table, and the data plane then installs only the necessary forwarding entries. If a link fails, routing re-computes and the new forwarding table takes effect — forwarding itself never "thinks", it only looks up.

Extra points worth mentioning

• Forwarding is data plane: it happens at every router for every packet and must be hardware-accelerated because line rates are very high.
• Routing is control plane: it runs in software (routing protocols) and runs much less often — on topology changes, periodically, or on policy updates.
• The two can be physically co-located in a traditional router, or separated as in SDN, where a central controller computes routes and pushes forwarding rules to dumb switches (control plane centralised, data plane distributed).
• Both rely on a common view of the network, so a wrong routing decision eventually produces wrong forwarding decisions for every packet on that path.