Final answer:
The protocol that elects only an active router and a standby router based on priority, with the active router having the highest priority, is HSRP (Hot Standby Router Protocol). GLBP and VRRP also offer redundancy, but they can support multiple standby routers, and LACP is not a redundancy protocol but a link aggregation protocol.
Step-by-step explanation:
The protocol that only elects an active router and a standby router, and where the active roster is the one with the highest priority, is HSRP (Hot Standby Router Protocol). HSRP is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. This protocol ensures that user traffic immediately and transparently recovers from first hop router failures, where two or more routers are programmed to use the same virtual IP address.
Other protocols like GLBP (Gateway Load Balancing Protocol) and VRRP (Virtual Router Redundancy Protocol) also provide redundancy but work a bit differently. GLBP allows for load balancing between a group of redundant routers, while VRRP is an open standard version of HSRP and does not support load balancing, but both can have more than one router in a standby state. Lastly, LACP (Link Aggregation Control Protocol) is not a redundancy protocol; rather it is used for bundling several physical links to form a single logical link for increasing bandwidth and providing link redundancy.