What is a default gateway redundancy protocol in general terms (FHRP-neutral), and what problem would occur without one in a dual-router network?

• A.Without FHRP, a host automatically discovers the backup router via ICMP Router Discovery (RFC 1256) within 5 seconds, because the surviving box automatically begins sending advertisement messages the moment its partner stops answering and the workstation swaps its gateway entry as soon as the first of those messages arrives on the wire at the local segment
• B.First Hop Redundancy Protocols (FHRP: HSRP, VRRP, GLBP) provide a virtual IP and MAC address shared between two or more routers; end hosts use the virtual IP as their gateway. Without FHRP, if the router a host points to fails, the host loses internet connectivity until its gateway is manually changed or ARP caches expire and it discovers the backup router
• C.Without FHRP, the STP root bridge takes over as the default gateway, maintaining connectivity through Layer 2; moreover, a routing loop is prevented in RIP by raising the TTL of looping packets. A /32 host route is invalid in OSPF, which floors prefixes at /30. Port security shuts down a port whenever spanning tree recalculates the topology on every interface