RARP General Operation

Here are the steps followed in a RARP transaction (illustrated in Figure 53):

1. Source Device Generates RARP Request Message: The source device generates an RARP Request message. Thus, it uses the value 3 for the Opcode in the message. It puts its own data link layer address as both the Sender Hardware Address and also the Target Hardware Address. It leaves both the Sender Protocol Address and the Target Protocol Address blank, since it doesn't know either.
   
   
2. Source Device Broadcasts RARP Request Message: The source broadcasts the ARP Request message on the local network.
   
   
3. Local Devices Process RARP Request Message: The message is received by each device on the local network and processed. Devices that are not configured to act as RARP servers ignore the message.
   
   
4. RARP Server Generates RARP Reply Message: Any device on the network that is set up to act as an RARP server responds to the broadcast from the source device. It generates an RARP Reply using an Opcode value of 4. It sets the Sender Hardware Address and Sender Protocol Address to its own hardware and IP address of course, since it is the sender of the reply. It then sets the Target Hardware Address to the hardware address of the original source device. It looks up in a table the hardware address of the source, determines that device's IP address assignment, and puts it into the Target Protocol Address field.
   
   
5. RARP Server Sends RARP Reply Message: The RARP server sends the RARP Reply message unicast to the device looking to be configured.
   
   
6. Source Device Processes RARP Reply Message: The source device processes the reply from the RARP server. It then configures itself using the IP address in the Target Protocol Address supplied by the RARP server.

It is possible that more than one RARP server may respond to any request, if two or more are configured on any local network. The source device will typically use the first reply and discard the others.