Which protocol operates at OSI Layer 3 and enables routing between Layer 2 networks?

• A. IPv4
• B. BFD
• C. BGP
• D. RIP

Answer: (A)

At Layer 3, routing packets across multiple networks relies on IP addressing. IPv4 provides the actual network-layer protocol used to forward packets between different Layer 2 domains (such as VLANs or separate Ethernet segments). Routers read the destination IP address and determine the next hop, enabling inter-network communication across diverse Layer 2 networks.

Bidirectional Forwarding Detection isn’t about routing decisions; it’s a quick-failure-detection mechanism. Border Gateway Protocol is a routing protocol that runs over TCP and is used mainly for inter-domain routing between autonomous systems, not the fundamental layer-3 mechanism that moves IP packets across networks. Routing Information Protocol is a Layer 3 routing protocol, but it’s one of several ways to learn routes; IPv4 is the core protocol that enables the actual routing of IP traffic between Layer 2 networks.