This post is in continuance to the previous post on DHCP fundamentals (Link) –
Now, we will understand DORA process in detail.
DORA process in DHCP stands for following message flows between the client and the server.
Below diagram depicts the message flow between the DHCP client and the Server –
Now let’s take a look what happens when these messages are exchanged between DHCP Client and Server. 2 key items should be kept in mind which are also important from interview point of view as well. These are –
- Network layer broadcast and
- Data Link Layer broadcast.
DHCP client sends out a DHCP Discover message to find out the DHCP server. DHCP discover message is a layer 2 broadcast as well as layer 3 broadcast.
Fields in DHCP Discover Message:
Hence from the above fields it is clear DHCP Discover message is a Network Layer and Data Link Layer Broadcast.
DHCP server receives the DHCP discover message from client and sends back the DHCP offer message with field information as below:
Hence from above field it is clear that DHCP offer message is a layer 2 unicast but still as layer 3 broadcast.
DHCP client receives the DHCP offer from DHCP server and sends back a DHCP Request message with following fields:
Above fields concludes that DHCP request message is also a layer 2 unicast and a layer 3 broadcast.
Once the DHCP client sends the request to get the Offered IP address, DHCP server responds with an acknowledge message towards DHCP client with below fields:
From above fields substantiates that DHCP Acknowledge is a layer 2 unicast but still a layer 3 broadcast.
For more details on the information you must get familiar with the DHCP header fields. Few important fields from DHCP header for our reference are as below –