Creating the Frame

What does a node do when it needs to create a frame and the ARP cache does not contain a map of an IP address to a destination MAC address? It generates an ARP request!

When ARP receives a request to map an IPv4 address to a MAC address, it looks for the cached map in its ARP table. If an entry is not found, the encapsulation of the IPv4 packet fails and the Layer 2 processes notify ARP that it needs a map. The ARP processes then send out an ARP request packet to discover the MAC address of the destination device on the local network. If a device receiving the request has the destination IP address, it responds with an ARP reply. A map is created in the ARP table. Packets for that IPv4 address can now be encapsulated in frames.

If no device responds to the ARP request, the packet is dropped because a frame cannot be created. This encapsulation failure is reported to the upper layers of the device. If the device is an intermediary device, like a router, the upper layers may choose to respond to the source host with an error in an ICMPv4 packet.

See Figures 1-5 to view the process used to get the MAC address of the node on the local physical network.