Routers are used to make forwarding decisions based on Layer 3 addresses such as IPv4 and IPv6.
Routers can build a routing table based on having a directly connected network, having a static route being configured on them, or using a routing protocol to learn routing information from other routers.
Routing protocols include interior gateway protocols such as Open Shortest Path First (OSPF) Protocol, Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Intermediate System-to-Intermediate System (IS-IS) Protocol.
Border Gateway Protocol (BGP) is an external routing protocol that connects service providers on the Internet today.