Which first-hop redundancy protocol that was designed by Cisco allows packet load sharing among groups of redundant routers?

GLBP was designed by Cisco to extend first-hop redundancy by assigning a single virtual IP address but multiple virtual MAC addresses, allowing each router in the group to actively forward traffic simultaneously. This provides both gateway redundancy and load distribution across up to four Active Virtual Forwarders (AVFs), which HSRP and VRRP cannot do natively.

HSRP (Hot Standby Router Protocol) is Cisco-proprietary but only one router is active at a time - it does not perform packet load sharing.

VRRP (Virtual Router Redundancy Protocol) is an open-standard FHRP that also uses a single active router and does not natively support load sharing across multiple routers.

VSS (Virtual Switching System) is a chassis virtualization technology for Catalyst switches and is not a first-hop redundancy protocol for load sharing.