what is dora process in DHCP and how it works?

This process of assigning the IP addresses by the DHCP server also known as DORA (Discover, Offer, Request, and Acknowledgement).

1) Client makes a UDP Broadcast to the server about the DHCP discovery.

2) DHCP offers to the client.

3) In response to the offer Client requests the server.

4)Server responds all the Ip Add/mask/gty/dns/wins info along with the acknowledgement packet.

Thanks.

 When the term "broadcast" is used, there are two types of broadcasts: 1) Network Layer broadcast and 2) Datalink Layer broadcast. For Internet Protocol, the standard broadcast DESTINATION address is 255.255.255.255. For Ethernet, the broadcast DESTINATION address is FF:FF:FF:FF:FF:FF. You have to think of the DORA process in terms of encapsulation and decapsulation, as well as switch behavior with frames it receives.

So let's say you've got two computers, a client host and a server host. The client host is the DHCP client and the server host is the DHCP server. Let's call the client host PC and the DHCP server DHCP.

Now, when the PC is turned on, the NIC has a MAC address but no IP address. So the PC tries to "Discover" the DHCP server by sending out a "Discover" packet. In that packet, the destination IP address is 255.255.255.255 and source IP address of 0.0.0.0 (because the PC doesn't have an IP address yet). That Discover packet is encapsulated into an Data Link layer Ethernet Frame. That Ethernet Frame has a destination address of FF:FF:FF:FF:FF:FF and the source address of whatever the MAC address of the PC's nic is, let's call it PC:MA:CA:DD:RR:SS (I know this is not a valid MAC address). Now, since the destination address is FF:FF:FF:FF:FF:FF, the first switch that receives the frame will, by definition, send that frame out of every single switch port, except the switch port that received the frame. Since the switch doesn't know (from it's CAM table), who FF:FF:FF:FF:FF:FF is, it forwards the Discover packet out of every single switch port, except the switch port that received the frame. Let's say the DHCP server is directly connected to the switch. So the DHCP server first receives the Ethernet frame with the destination address of FF:FF:FF:FF:FF:FF (which by definition, means itself), and a source address of PC:MA:CA:DD:RR:SS. So when the DHCP server receives that Discover Packet, it replies to it by sending the Offer Packet. Since the DHCP server has no idea who was contacting them (since PC currently has an IP address of 0.0.0.0), the Offer packet has the destination address of 255.255.255.255. But the DHCP server remembered the MAC address of whoever it was trying to contact them, so the DHCP server sends out the Offer packet in a frame with a destination address of PC:MA:CA:DD:RR:SS and uses it's own source address of DH:CP:SE:RV:ER:MC (again, I know this is not a valid MAC address). So the Offer packet is sent from the DHCP server to the PC as a unicast not a broadcast. So the switch receives the frame from the DHCP server and forwards it to the PC which receives the Offer packet. Then the rest of the DHCP process of R and A occurs.

So that's why the "...the first two messages in the DHCP are broadcast messages..." refers to Network Layer IP broadcast messages (destination IP address of 255.255.255.255). Then "...only the first message (the Discover message) is a LAN Broadcast..." because only the Discover packet's frame had the Ethernet destination address of FF:FF:FF:FF:FF:FF (therefore a LAN broadcast) and the Offer packet's frame had the Ethernet destination address of PC:MA:CA:DD:RR:SS (therefore a LAN unicast).

I hope this helps.

DHCP (D)iscover
DHCP (O)ffer
DHCP (R)equest
DHCP (A)ck

Step 1:

Your computer sends a "Discovery" request, asking for it's IP information from any listening DHCP servers.

Step 2:

Any listening DHCP servers will "Offer" their configuration information to your workstation.

Step 3:

You workstation chooses the best lease then "Requests" that lease from the corresponding DHCP server.

Step 4:

The DHCP server you requested the IP configuration information from then "Acknowledges" your request and leases you the IP configuration information.

The DORA process in DHCP works as folows.



D ----------> Discover

O ----------> Offer

R ----------> Request

A ----------> Acknowledgement



1. DHCP automatically discovers the system

2. Server sends offer to the client system

3. Client sends request to the server system

4. At lats Acknowledgement is send by server to client system

1) Client makes a UDP Broadcast to the server with a DHCPDiscover, or Discover packet.

2) DHCP offers to the client.
The server sends a DHCPOffer including other configuration parameters (DHCP Options) for the client per the servers configuration file

3) In response to the offer Client requests the server.
The client replies DHCPRequest, unicast to the server, requesting the offered address.

4) The server sends DHCPAck acknowledging the request which is the clients final permission to take the address as offered. Before sending the ack the server double checks that the offered address is still available, that the parameters match the clients request and (if so) marks the address taken.

A good reference for DHCP information is Droms and Lemon "The DHCP Handbook".

DHCP (D)iscover
DHCP (O)ffer
DHCP (R)equest
DHCP (A)ck

Step 1:

Your computer sends a "Discovery" request, asking for it's IP information from any listening DHCP servers.

Step 2:

Any listening DHCP servers will "Offer" their configuration information to your workstation.

Step 3:

You workstation chooses the best lease then "Requests" that lease from the corresponding DHCP server.

Step 4:

The DHCP server you requested the IP configuration information from then "Acknowledges" your request and leases you the IP configuration information.

Dora=
d-discover
o-offer
r-request
a-acknowledgement
=========
dhcp first discovers the computer and it then offers the computer to assign an ip address to i.then the computer request for the ip address when it accepts the offer from dhcp..when dhcp assigns the iip to the computer then it sends an acknowledgement that it had been assigned the ip address ...this is process of DORA....

The DORA process in DHCP works as follows.

D ----------> Discover
O ----------> Offer
R ----------> Request
A ----------> Acknowledgement

1. DHCP automatically discovers the system

2. Server sends offer to the client system

3. Client sends request to the server system

4. At lats Acknowledgement is send by server to client system

check this vedio this might help:
http://www.youtube.com/watch?v=0Dp7YoR0SLE&feature=g-user-u