For a mixed-vendor network, which redundancy protocol prevents traffic interruption if the default gateway fails?

VRRP (Virtual Router Redundancy Protocol) is the correct answer because it is an open-standard First Hop Redundancy Protocol (FHRP). Being an open standard means it can be implemented across devices from different vendors, making it suitable for mixed-vendor networks. If the active router fails, a standby router seamlessly takes over the virtual IP address, ensuring continuous network connectivity. FHRP is a category of protocols, not a specific protocol itself, so it's too general. HSRP (Hot Standby Router Protocol) and GLBP (Gateway Load Balancing Protocol) are Cisco proprietary protocols. While they provide similar redundancy, they are primarily designed for Cisco-only environments and may not be compatible or fully functional in a mixed-vendor network without specific vendor interoperability features.