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New Member

Reg: DHCP

Dear Experts,

I had query in DHCP. I understood well working of DORA process in single broadcast domain( i.e DHCP client and DHCP server in the same broadcast domain).

How DORA process will work if DHCP client and DHCP server are in the different broadcast domains. I know it will work if we configure helper-address in the router interface of client broadcast domain. So at that time the router will send unicast meesage to the DHCP server. But how DHCP offer message will reach the DHCP client from DHCP server( First i want to know DHCP offer message from server is broadcast or unicast at both layer 2 and layer 3 levels).

Please Give me in more dertail for understand purpose.

Regards,

Janardhan

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Accepted Solutions
Purple

Reg: DHCP

Hi,

it's unicast from the server to relay but then from relay to client it is broadcast or unicast depending on broadcast flag in the original dhcp discover.

Regards.

Alain

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9 REPLIES

Reg: DHCP

Hi,

If DHCP clients and servers on the same subnet the the communicate via UDP broadcasts.
If the client and server are on different subnets, a DHCP Helper or DHCP Relay Agent may be used. Clients requesting renewal of an existing lease may communicate directly via UDP unicast.


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Regards,
Naidu.

New Member

Reg: DHCP

Hi Naidu,

Thanks for your prompt reply.!!!!!

I know in this scenario, we will use helper or relay agent to get IP address.  If it is the first time, how DHCP offcer msg will come to the client from DHCP server( Because the client doesn't have IP address i.e 0.0.0.0, right???)

Please help me to understood this scenario...

Regards,

Janardhan

Purple

Reg: DHCP

Hi,

I suggest you read this: http://www.tcpipguide.com/free/t_BOOTPRelayAgentsForwardingAgents-3.htm

Regards.

Alain

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New Member

Reg: DHCP

New Member

Reg: DHCP

hi,

i just tried it here with my existing equipment(no cisco) and made a trace with my network analyzer.

the result was surprising to me.

dhcp offer:

ethernet frame:     source mac: mac of the dhcp server;               dest. mac: FF:FF:FF:FF:FF:FF

ip packet:               source ip: ip of the dhcp server;                      dest. ip: 0.0.0.0

but there was an additional packet or protocol or dont know how to call it.

bootstrap:               dest. mac: mac of the pc

                               dest. ip:     ip address which was offered to the pc by the dhcp server.

i actually thought that unicasts are used.

florian

Purple

Reg: DHCP

Hi,

it's unicast from the server to relay but then from relay to client it is broadcast or unicast depending on broadcast flag in the original dhcp discover.

Regards.

Alain

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New Member

Reg: DHCP

Thanks Alain...

Some waht  clarrified with your suggested link.  Is there any document on this to fully undetstand.

Regards,

Janardhan

Purple

Reg: DHCP

Hi,

http://www.faqs.org/rfcs/rfc951.html

http://tools.ietf.org/rfc/rfc1542.txt

Regards.

Alain

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New Member

Reg: DHCP

Hi,

I think it is same broadcast as DHCP client and Server.

Regards,

Janardhan

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