Which protocol should be run on the LAN side of two edge routers (that are terminating primary and backup WAN circuits) to provide quick failover in case of primary WAN circuit failure?

VTP (VLAN Trunking Protocol) manages VLAN configuration across switches and has no role in WAN failover or gateway redundancy.

STP (Spanning Tree Protocol) prevents Layer 2 loops in switched networks and does not provide IP gateway redundancy or WAN failover capability.

VRRP (Virtual Router Redundancy Protocol) creates a virtual IP address shared between two edge routers, with one acting as the Master and the other as Backup. When the primary WAN circuit fails, the backup router can be configured to preempt and become Master, providing LAN hosts with uninterrupted default gateway reachability. This failover is driven by tracking the WAN interface state, enabling fast, automatic switchover without requiring any reconfiguration on LAN hosts.

RIP is a distance-vector routing protocol used for route exchange between routers; it does not provide first-hop redundancy for LAN hosts or quick gateway failover.