What protocol or technology uses a standby router to assume packet-forwarding responsibility if the active router fails?

The protocol that uses a standby router to assume packet-forwarding responsibility if the active router fails is HSRP (Hot Standby Router Protocol). HSRP is a Cisco-proprietary redundancy protocol for establishing a fault-tolerant default gateway. It allows multiple routers to work together to present the appearance of a single virtual router or gateway to the hosts on the local network. If the currently active router fails, the standby router takes over the IP address and continues forwarding packets.