Hello Juan
When the DHCP server is in the same subnet, the following communications take place:
- DHCPDISCOVER is broadcast on both layer 2 and layer 3 (MAC and IP)
- DHCPOFFER as a response to the discover is unicast. It uses the MAC address of the original sender as the destination MAC and the proposed IP address as the destination IP (even though the DHCP client does not yet have an IP address assigned. This doesn’t matter since communication is happening at Layer 2 for now since we are on the same subnet)
- DHCPREQUEST is also broadcast on both Layer 2 and Layer 3. Take a look at this sample wireshark capture of a DHCP Request. Notice the destination MAC and the destination IP are broadcast addresses:
- DHCPACK from DHCP server to client is also unicast.
Now in the case of a relay agent, refer to the diagram from the lesson. All traffic between the relay agent R1 and the DHCP client H1 remains the same as that described above. However, as stated in the lesson by Rene, the traffic between the R1 and the DHCP server that exists on another subnet is unicast.
I hope this has been helpful!
Laz