Which routing protocol helps a host discover the IP addresses of active routers on its subnet by listening to router advertisements and solicitations?

• A. ICMP Router Discovery Protocol (IRDP)
• B. STP
• C. VLAN hopping
• D. Switch Spoofing

Answer: (A)

The host needs a way to learn which router to use as its gateway on the local subnet, and it does this by listening for Router Advertisement messages from routers and by sending Router Solicitation messages to provoke those advertisements. This interaction is defined by the ICMP-based Router Discovery Protocol (IRDP), which allows a host to discover the IP addresses of active routers on its subnet and learn the default gateway to reach destinations outside the local network. Spanning Tree Protocol operates at the switch level to prevent loops and isn’t used for discovering router IPs. VLAN hopping and switch spoofing are security techniques that don’t provide router discovery functionality.